I decided to break up with him
you broke up with him
he moved on
time for you to do the same with your life
if the guy wants to spend time with his child, arrange that, be supportive of their relationship
get on with your own life
Once you break up with someone *BOOM* the chances are pretty good that the previous relationship is over and done with.
The next time you think about breaking up with someone but think you don't want it to be permanent - don't break up with them. Get counselling/work on the relationship.
Separately - he is in a new relationship. It really is NOT your place to consider interfering with that. It makes you a total asswipe to even consider it.