Sole legal is actually still very common. It however does not keep dad from having visitation. If he has a visitation schedule, he is still entitled to it.
The college issue is a big debate, as judges can force divorced parents to pay, but not married parents. given that the judge is counting the college education cost of son as support, he would probably consider your husband respoinsible for college of the 16 year old as well. So once he is no longer paying for the son, the mom can take him to court for part of the daughters share.