Honestly, he would develop more in the G League than playing another year in the Big Ten. Playing in the G League, he would be able to be around professional coaching as much as he wants, and not have to deal with the NCAA limitations on practice time and having to go to class.
I have no idea if he's close to graduating, but maybe he sees he has a ceiling for himself at Nebraska, and he would be better off developing in the G-League or give it a shot overseas. There is nothing wrong with having options.