His decision should be made based on his prospects in the NFL. If he's confident in getting drafted, he should leave...whether he graduates or not.
If he isn't confident in being drafted...AND, he thinks he can improve on that evaluation next year by coming back...he should come back.
Unless he's a 3rd round pick or better, what is the real difference in pay?
The guys that get drafted get paid just like anyone else. You have to be on the 53 man roster to continue to get paid like you agreed in your contract, right?
Once they get cut, they don't get paid. So if he gets drafted...good, but if he is a udfa, the league minimum is still good pay...if you stay on a team.
I say, declare for NFL, if I were looking out for his interests.