I just want him to be the person I met
I don't want this to sound harsh. But you were both still at school when you met, and you are still not adults yet. You have had a baby and while you're still living with your parents, you've had to do a lot of growing up in a short space of time, to become a mum.
Your boyfriend hasn't gone through that same physical process, and as he isn't living with you, maybe doesn't quite "get" the whole becoming-a-parent thing, and hasn't matured as quickly as you have.
As people get older, they do change. The difference between who you are when you're at school, and who you are when you've left, is huge. (Thank goodness, because I was an utter brat at school
)
Unfortunately, it may be that he was not ready to be a dad, and he wants to continue doing the kind of thing you can do when you're not a parent - go out with his mates, etc.
I think the best thing you can do is sit him down with him and have a serious talk about where he sees things going, and where he wants to be. Whatever happens, you want to make sure he is still in your DS's life as his dad.