Mo is not a 300 lbs man in the same way most people are 300 lbs. He is out of shape but he isn't like trying to get the guy from Mike and Molly to be able to move a bit. Mo could get in good enough condition to play in spurts over 1 off season. You see it all of the time in college basketball with guys like Nix, Pittman (Texas a few years ago) and even Kevin Love in the NBA.
It's certainly possible to go from out of shape into good enough shape to contribute in college basketball.
Does he have the discipline? I don't know how anyone could make that guess. He is obviously not a health nut, but he didn't look grossly out of shape until this season (coming off a serious injury), so he could possibly have the discipline, a lot of guys figure it out later. Does he? Who knows?
In short, Mo cant go from his current shape into being an iron man or anything in one summer, but he certainly can get himself into decent enough shape to contribute in college basketball.