when I was 18 I had a painless abscess on the roof of my mouth to the right of my pallet. Mild discomfort to press on it but no real pain. Turned out
I had a tooth that had been damaged at some point and the root and nerves had died...has your son ever hit the front of his mouth (a fall or bike
riding or anything?)
had to drain it and get a root canal was a pain in the arse but not terribly painful.
Cankers are painful so if he's not complaining of terrible pain...I'd suspect tooth injury at some point and a simple Xray would show if that was
it.
Not to worry, I'm sure he's fine

As a Mom I understand the worry when the kids are away from home but as I said unless he's in real pain I
wouldn't worry.
best of luck!