The fact that he's a father wouldn't make me dislike him, but the way he is talking about the mother would. He got this girl pregnant, has left her on her own, and now dumps on her. Not a good sign. Him wanting custody might be more about him not wanting to pay child support to the mother than anything else.

Since you're in your early 20's, I think you can do better than this guy, who sounds like sour grapes to me. He's going to spend the next 18 years resenting this baby's mother. He's got a bad attitude and you'll be dragged into a lot of ugliness, if you stay with him long term.

When he gives you all the evidence for how no good this other girl is, remind yourself that you will be hearing only his side. That girl's got a side to.

If I were you, I'ld pass on this guy. He's already screwed up his life. You don't need that. At your age, you really don't need to become a stepmother to a child in the middle of a custody battle.