I would keep your son away from him, and explain to him little by little as he grows up what's happening. If he wants he can write letters to the boy and you can send him letters and pictures.
Otherwise your kid will grow up thinking it is normal and okay and acceptable to have a father in prison for doing/selling drugs. That it is okay, normal and acceptable for the incarcerated man to string you along like he has.
Perhaps when he gets out/if he gets out and proves for a few years that he is clean and living a good life then he can get involved. As for now do what's best for your child and I don't think daddy is.
If you were alone I would've told you to do whatever you want. When there's a kid, you've got to think twice and it sounds like you are which is a great thing.
I'm sure you feel bad for him because you have had a long relationship. He made his bed. He knew it, and chances are he won't change.