Gotcha; I had worried about having the same feeling, but the ship is SO large, and the activities so varied (especially as a new cruiser) that you kind of forget you are "stuck". Well, and having port days helps too since you
can leave!

We booked a four night due to fear on our first, and were sad to leave the ship so soon. Does he often feel stuck in malls, or other places? If not, then he very likely has nothing to worry about. This is NOT like being on a little boat, or plane even. You have a whole nice room to retreat to and relax and forget where you are, and fresh air for days on deck even if you don't get a verandah. I will , say that if he is a very active person a larger RCCL ship with more outdoor activities like rock climbing may have the edge over
DCL if that would help him feel less stir crazy.
The problem with being concerned about stomach issues is the very act of worrying can easily CAUSE stomach issues... I will say that the ships cook meat to oblivion, so aside from the ever present danger of picking up a stomach bug, which can happen, the odds of getting food poisoning onboard is probably lower than dry land! I think you're right to not press the matter *too* hard, since he will decide how he feels about it and that can affect his body and mind. Conversations with friends and folks he trusts who enjoy cruising can't be beat in that case.