My advice:
Do not try to save the world, or this person’s soul, you have not proven that you are equipped to change someone who didn’t ask you for help. The fact that you know him does not qualify you to change the situation, though you can easily make it worse.
Stay away from him as much as you can. Associating with him will not do your neshamah any good.
Let the mother raise the child and hope for the best.