but I want to raise him myself once I'm able to get rid of my postpartum symptoms and once I'm stronger as a single parent
I understand that you want to raise your son. I think you should explain this to your in-laws as soon as possible, so that there are no misunderstandings about your intentions (and depending on how your marriage and your relationship with your in-laws develops, you might also want to get legal counsel, but for the purpose of this answer, I'm assuming that everyone's getting along).
I also think you should take steps to figure out what you can do in order to be ready to take your son back. It sounds like currently you feel unable to do so, and so it probably makes sense for your in-laws to provide a stable environment for your son. But if you want this to change in the (near) future, you should have a plan on how this change could come about. It doesn't seem fair to your in-laws and your son to leave them in the dark about how long your son is going to be staying with them, especially if they would like to keep him with them, so I'd try to get together with them and work out a rough timescale as soon as possible.
This also involves finding answers to some important questions, such as do you have any support besides your husband? Can you get him to take an interest in the baby? How bad do you feel? Bad enough so taking care of a child is unwise? Have you arranged any counseling for yourself, and possibly also family counseling for yourself and your husband? Is there a horizon for when you might feel strong enough to take care of your son again? Do you have an idea what you could do to get better and stronger? Do you think your marriage will recover, or are you going to be a single parent? ...
Also, it strikes me that you're seeing this as an either/or situation - either your son lives with his grandparents or with you. But I don't think it has to be. Maybe you could arrange taking care of your son for a day or two every week at first, and increase that once you get better. Of course, this is only possible if you live close to your in-laws, but if you don't, that's probably one of the things you should change soon. It enables you to be a part of your son's life and shows to your in-laws that you want to be a part of his life, even if currently you can't take care of him 24/7. I don't think kids have a problem with multiple care givers as long as they all take an interest in them and are reliably there for them. In fact, I think it might be a win/win solution for everyone involved, including your son.
or bear the sacrifice and wait until the day my son asks to live with us
I don't think this is likely to happen by itself. If your son is being raised by his grandparents from a very small age, they become the persons he attaches to and identifies as his immediate family. Kids (like most people) tend to prefer knowing the circumstances they live in, it makes them feel secure, and that can trump living with their natural parents. So if he's been living with your in-laws from a very small age and you haven't been there, I think it's unlikely he'll develop a wish to change his living conditions anytime soon - on the contrary; the longer you wait to resolve the situation, the more attached he'll be to his grandparents, and the less willing he'll probably be to give that all up and go live somewhere else he isn't familiar with, unless his grandparents can't provide him with what he needs. That isn't to say that you won't be able to take him back later on, but it will certainly be harder both on him and on your in-laws, especially if you haven't kept up regular and frequent (meaning close to daily) contact.