He should be required to pay a licensing fee to the University for using its Name, Imagine, and Likeness like any other retailer out there now that he has left. Those contracts with players outta be contengient on their enrollment with said university and if you leave you gotta pay back the licensing fee. They are supposed to already document the sales and deals with the university, so attaching a value to it should be pretty easy.
The university invests in these guys much more than they are getting out in a lot of cases. You only need to look at the P&L of most athletic departments to understand that. Someone has to step in and regulate this crap because there is no way all of the paperwork is being properly processed. Next thing we'll have are Rhodes scholars wanting NIL for their advancement with their studies and such that universities trademark.