This is something you and your husband cannot solve... it must be solved by all three of you: you, your husband, and your son as a family. I cannot pretend to be in a position to tell you exactly what to do, only to point out things I consider to be "facts" in the situation, and let you decide if you agree with my claim. However, "self love" is not something that you can cause somebody to do, you can only encourage it. Encourage it until they eventually decide to take the first step, and then encourage them for doing it.
(So, in that light, I suppose my first sentence is wrong. There is another way to solve it. Your son can solve it on his own. Studies of people in terribly dire straights show that it is possible to develop self love in any scenario. That being said, its quite clear that, as parents, you don't want to leave him on his own to discover this, so we'll focus on the family as a whole, and how the family can work with this).
I do think it's important for you to understand what your punishment must be doing to your son. I will not question corporal punishment; others have done so, and frankly I believe the nuances of how punishment is delivered are more complicated than people believe. However, if I may be so bold as to use your own word from that sentence, I think I can paint a picture from your son's perspective that you might be relate to:
We are pretty strict parents and believe in corporal punishment
especially since my son doesn't have anything left to take away since
he's always in trouble for something.
Emphasis mine. If we ignore everything else except the fact that you find a need for punishment and this bolded section of the sentence, we can see a fundamental structure of a relationship. It shows that you have taken away all the easy things from him, and he's misbehaving still. Thus, you are actively seeking out other things to take away from him, and relying on him to go find those things so that you can take them away. One thing I have found true about punishment: you can always guarantee that you take something away from them, but you can't always choose what it is. Thus, it should not be a surprising if, after rolling the dice this way for a long childhood, one day your son offered his self love to be taken away and you took it. Neither party may have even known that's what's being offered. Punishment can be a tricky business from both sides. I know that, in the face of punishment, I have held up things I value most to take the blow for me before I even know I did so. (There's something to be said for the lyrics of Big Yellow Taxi, "You don't know what you've got till it's gone.")
I believe it is clear from your words that you feel it is necessary to be able to punish your son, so the good-fairy attitude of "just love your son, everything will be all right!" will not sit well with you. The devil's in the details anyway.
There honestly isn't a clear path to solve this problem. If there was, society would have abused it to let us hurt each other more, knowing we can always follow an easy path back to self love. You will have to seek out your own path, and that path will be rocky. However, there are some milestones along the way which I have found universal enough that they bear mentioning.
The most important step is to be aware of your son. Listen to his words, listen to his body language, listen to everything. If your son is in dire enough straights that the phrase "doesn't love himself" is warranted, the spark of life you will want to fan is going to be faint and hard to spot. He will hide it from you as best as he possibly can; he won't make the same mistake twice -- he wont give you an opportunity to punish him through it. You will have to look hard to find it. When you do, you will have to make a very hard call as parents who have relied heavily on punishment. You will have to decide not to lay your hands on it, literally or metaphorically. You will have to learn how to support it from afar until he learns to trust you enough to bring it out into the open. You will have to let him have it, no matter how angry you are at him. Self love is hard to develop once you lose it, and you need to be able to listen to your own inner light once it starts glimmering.
The subtle techniques needed to support such a glimmer from afar are hard to learn. They are different for every parent child pairing, so you will have to do some soul searching to learn a bit about it yourself. However, there are some general patterns that you can manage.
The first is to only take that which you give. Instead of relying on striking his physical body (which technically, yes, you gave, but I think you understand why I ignore this technicality), you need to give him things which can explicitly be taken away -- privileges. If he has nothing, you cannot punish him, as you have found out. However, given the scenario I see described in the question, I recommend giving them in a particular structure: only give him things in a way which, if you take them away, hurts you as much as it hurts him. Think of it as a voluntary limitation on your own power in the name of making the family stronger. "We will respect the sanctity of your room, but you are obliged to call us whenever you go somewhere". This limit does two things. One is it restrains you (which, believe it or not, can be a good thing). Second, it gives him something to see and feel in you. Let him realize (on his own) just how much each of these new punishments hurt you. Don't fake it. He needs to see the real you, even in pain, to appreciate that what he is enduring is not arbitrary from your perspective.
One approach I would consider (disclaimer: I've never tried it, but it has the aire of truth to me) is to set up an agreement. Set up a set of obligations on both parties as part of the agreement. Try to make them balanced. Then, as a key step, as part of the agreement, declare that either party may revoke the agreement at any point in time, for any reason. This agreement is ephemeral. It only functions so long as both parties want it to function. This gives him control over you, which may be something he needs, but also puts a check in place to prevent him from abusing it. Explicitly do not state what could cause the revocation as part of the agreement. This isn't an agreement between him and a piece of paper with some rules, it's an agreement between you and him. Both parties deciding that working together is more beneficial than working apart. You may go through 20 or 30 agreements, each rapidly fizzling out. This is okay. You don't even have to make each agreement identical (indeed, you will find that they're never identical, even if the words are identical, because the parties will have adjusted their opinions on the revocation clause). Make multiple of these if needed. But show that working together as a family is better than working apart. (And as a note, if you do it right, your son will use this as ammunition against you, severing agreements if you rely on corporal punishment. Believe it or not, this is not a bad thing. It's a key step in standing up for himself, which can lead to self love if you do it right).
To close, I have to go back to the good-fairy argument because, frankly, there's no way to solve this problem without some input from her. Your son is depressed and lost his self love. He needs the softer side of the family, and you are the ones who can give it to him. If some of that depression turns to rage, and he lashes out, try your darnest to take what he gives, turn it around, and make something good out of it. There's a whole self-help section of the library dedicated to this skill, so I wont pretend its easy to do. I wont even pretend I know how to do it, per se. However, if you can take something he unleashed out of rage or sadness, and turn it into a glimmer of hope and beauty in a way that he never thought he could see, that just might be the thing he needs to find self love. You can show him that even a nugget of ugly emotion can be polished into a diamond. You can even encourage him to learn how to polish them. And, if all is right with the world, he can make the choice whether he polishes them or not.