Then he should have worn a raincoat all those years ago.
Child support is his responsibility.
Don't you dare feel bad for requiring that he live up to it.
At this rate, Mama will write the checks for his sorry butt every month and he'll continue to act like he's 14 anyway.
BTW - I wouldn't talk to her if I were you. Not about anything you feel regarding her son. 10 dollars to a donut it's going right back to him and fueling the fire...