he graduated, so he can enter anytime. Normally grad transfers have to enter by May 1 to be eligible for the fall. I am honestly not sure how all these rules interact in order to know if he has an exception. For example, if Nebraska stripped his scholarship for team rules violations, I believe he would be immediately eligible.
He has already redshirted, so the clock will tick another year this year regardless of whether he plays - he would lose a year of playing eligibility if he can't play this fall.