After all, there are only boys in the boys' dormitory, so you don't need to care too much about your image. Many boys' dormitories only wear a pair of underwear in front of their roommates. Maybe the new roommate didn't do it when he was not so familiar, but it will definitely happen after he is familiar.

If they don't do this, you can take the lead and they should follow suit. They may not do this because they are not familiar with it at first.

In short, if there are no girls in front of all boys, you can really go shirtless regardless of your image.