In one part of your description, you said
he is constantly mean to my baby!
This may be reflective of something that is part of the issue. Remember that until a couple of years ago, the one who is now 5 was your baby. He has lost that position, at the very least in his mind.
Sometimes dealing with a younger sibling entering the picture can leave big brother/sister feeling as if they have lost a place in the parent's heart- and think of how difficult that would be to deal with, even as an adult. Yes, clear boundaries must be set so that he knows this behavior won't be tolerated, and the behavior needs to have consequences. Often, time outs or loss of a privilege or favorite toy are more effective in these cases than spanking, and certainly more effective than just verbal scolding.
But aside from that, here are a few things you might want to try.
Look for times that he is acting nice, and make sure he gets a
significant "attaboy" (at very least verbal praise-- "thank you for
acting so sweet. Mommy is so proud of you!") for those times you can
catch him in the desired behavior.
Try developing a few activities that are just for you and him. I know
it's hard as a single mom, but so important that he gets some
individual positive attention from you. Maybe during baby's nap or
after he has gone to bed, let big brother stay up an extra 20 minutes
each night and have some special story time or a special big-boy
snack that you prepare together. Really anything that can be an
activity for just the two of you, especially if it's something the
younger one isn't yet "big enough" to do. This can have the added
benefit of starting him on seeing the benefits of being older.
On activities that you need to do together with both kids, make the
older child your special helper. Give him a few simple
responsibilities and make sure to let him know how much help it is
when he does xyz task that you have given him.
Make sure and give him time/space to talk to you about his feelings
without condemning him for it. It's okay to redirect, but don't scold
him for how he feels. What I mean by redirect is to get him talking
about how he feels about not getting the attention he wants, rather
than focusing on being angry at baby brother - that kind of thing. But
the main thing is to let him feel that you understand where he's
coming from, not that you're angry with him for feeling that way.
Once he is feeling like he is still the apple of your eye, and that he is being heard, over time chances are his feelings about his brother will evolve into more positive ones. And there's a great article from "Psychology Today" about this.
As for the ADHD dx and medications - I strongly recommend talking to the doctor that diagnosed him about you not wanting him on meds, and if there's a different way to approach his treatment. If his doctor is unwilling to consider other options, it would be worth getting at least a second opinion. You are your child's advocate in these things and if you don't feel the treatment plan is the best one for your child then you should absolutely speak up. Sometimes our instincts about these things as parents can be off base, but most of the time we know when something like that doesn't seem right for our child. And when we are off base, our doctor or other health professionals should be willing to explain things further in a way to help us understand why they are recommending a particular treatment protocol.
Anyway, I hope this is all helpful.