I agree with Silvia, DRs seem so quick to just put these kids on medication and I've noticed it more so with boys than girls. Your son has gone through a lot in his short life so far and may have a lot of pent up emotions and I wouldn't blame him. And little kids don't have the tools to properly express themselves about things like this. He may feel abandoned by his mom, his dad married you (most kids feel like the step parent is a replacement) and has a baby sitter to battle for attention. As far as being told 'no' and stuff like that, how was his behavior handles before? Was he pretty much allowed to do what he wanted or have the rules recently been laid down in the house? I would suggest this, you and his dad sit down with him at the same time. Make sure the baby is either sleeping or have a friend watch her at their house. This way she can't be a distraction and the both of you can focus on him. Ask him questions like how he feels, feels about the baby, his room, etc then lead into the harder ones about his mom. He may not talk and that's fine. But just let him know that he is able and capable to come to either one of you to express himself and he won't be in trouble for it. You might also want to take this time and clearly set the rules of the house, what you expect from him, and what will happen if the rules get broken. It will probably be a power struggle at first, but if you both stand your ground he'll figure out that you mean business.
In the mean time, follow the Dr's orders about weaning him of the meds and go from there. Hugs to all of you:) I'm sure this is a rough time for everyone.
S.