With a child being born, the only way to void the relationship (as opposed to simply divorcing, which he may do) is that he can get an annulment IF it can be shown that he is not mentally competent: only mentally competent persons can marry. Mild intellectual disability is not necessary enough: again, he'd have to be not competent to manage his own affairs to have grounds for an annulment. If he has legal guardian due to lack of competency, that is probably enough.