off-campus house with older/upper classmen?
Water, Barley, Rice, Malts, Hops, and Yeast could also be an explanation for weight gain 
But I do agree about looking out for supplements - as long as he's educated about which ones are good and bad from someone at the school like the trainer who is trained to know. The list does change and the NCAA does have some fairly comprehensive testing. It's always 'someone else' who gets caught until you become the next someone else. Those protein supplements along with regular lifting surely can add weight properly especially if they've never been used before. I had two boys (baseball players) gain some significant weight/muscle mass when using them. Stuff smells awful IMO, but it worked. Funny how they'll have that, but have avoid certain food and vegetables throughout life because it looks funny or smells bad, but give them a protein supplement and they're all over it.