My boyfriend has been divorced for 4 years and his exwife will not let him see the kid. He is in one state and the children is in another. They are young adults (17 and 15) and she won't let them come to visit. What can he do?

What does the divorce decree say regarding custody and visitation?

If she isn't abiding by that, move (in the court which granted the divorce) to have her held in contempt of court.
If the decree is silent regarding custody and visitation (or doesn't set forth a specific visitation schedule), move to amend it.