Checking for gas is a little difficult if you arent used to the “normal” feeling of your buns belly. I have to check quick silvers daily, but even i sometimes can miss it. Best way to check is sit your bun flat (if you hold him upright like we stand, it can feel like gas when it isnt. At least with my bun, it does.), have one hand on or around him to hold him steady if hes a wiggler, and gently rub his belly with your other hand (i generally just use 2-3 fingers). You do not need to press hard. Just lightly rub and feel if its soft, or if it feels hard. If it feels hard, it is possible there is gas. This is how my vet taught us. Other signs of gas could be grinding teeth hard or shifting as if trying to get comfortable.
Is he fixed? Is the other bun and him allowed to play together?
I know one difficult change i made was working night shift. I had to let my buns out at weird times in the day. At first, they were both pretty sleepy, but they slowly grew accustomed to it and now will try to rip through metal if im more than a few minutes late on letting them out. Have you kept a steady time on them being out or is it different every day? I know buns can be pretty scheduled and it can effect their mood.