That depends.
Did he even know where the child was? My ex tried her best to keep my child as far away from me as she could. The only thing she wanted was the support payment.
If she kept good relations with him over the years, I think he should help her, and he definitely needs to be in his son's life, but if she was a witch toward him, he owes her nothing. In fact, the lawsuit has "witch" written all over it. An approach based on honey instead of vinegar would probably get results.
Good point.