I looked into this extensively when I had someone begging me to the point of harassment to let him come work for us to "gain experience". (We had reason to believe he was setting us up for industrial espionage, but that's another story.) The bottom line is that unless he is receiving school credit for it, it's virtually impossible to legally have an unpaid intern. You would have to be working THROUGH the school and most likely paying the school (who would then provide him with a stipend) to do it legally. There might be another way to go about it, but it's a complex issue.
It would be far, far simpler to hire him as a minimum wage employee for a limited time, maybe for the course of the semester.
Two things I am tired of typing: 1.) A wrongful termination does not mean that you were fired for something you didn't do; it means that you were fired for a reason prohibited by law. 2.) The above answer, whatever it is, assumes that no legally binding contract or CBA expressly says otherwise. If it does, the terms of the contract apply.