I am not a parent, so if you don't find me credible that's alright. But any interestion between two people is a relationship, be it parent/child or not. One of the important things about any serious relationship (and by serious I mean those relationship which are more than task related i.e. those which are usually temporary and get us to a short term goal, for instnace the relationship that any Wendy's employee has with a customer) is that, among other things, communication must be open and honest.
For purposes of argment lets assume most people who go to jail are guilty. Good people don't go to jail! Jail is there to punish bad people who have broken laws (Wheather this works or not in a whole other issue!). As you established in your message, you feel he is a bad person too.
Back to open and honest comminucation... it's agreed that he is a bad person. You need to be honest about it. Dad cannot be an exception to this rule. If going to jail is bad, than Dad is too. Otherwise, what happens if someone starts thinking "My Dad is in jail, but that's ok because he's my dad." or worse yet "My Dad isn't a bad person, so being in jail isn't bad."
Second of all, a good parent begins with being a good person. Normally, I don't support single parenting. However, when one party isn't willing to be a responsible adult and do things that are necessary to provide their child with a good life, sometimes there are no other options. Clearly, he wasn't willing to do/not do whatever it was that was needed for him to stay out of jail, and be a parent and provide for his child. That being said, you say he will never change, in which case you should never let him near his child, until he proves that he can be a respinsible adult who is willing to give his child the best.