Stress and diet changes can affect the rabbit tummy, but very often we just dont know what causes tummy problems.
I hope you’ll soon see poop. Since he is esting, chances are good he’ll poop. Try to get extra fluids in him by serving wet greens. You can make hay tea to entice him to drink more (pour hot water on some hay, let sit until cool, sieve off the hay or leave some hay in, and serve in a bowl.
If he doesn’t poop and starts exhibiting signs of pain (wont eat anything, sits very still), you should contact a vet, because it could be a partial intestinal blockage, which is serious. If the upper part of the belly seems round and “full”, don’t massage it.
A vet can give emergency care if needed, or “simply” give sub q or iv fluids to hopefully hydrate whatever the blockage is so it starts moving. A vet can also give more heavy-duty pain meds than we can at home.
It’s very good that he is moving around and eating. Maybe cut out the carrot and other sugary treats for now, since sugar can cause excess gas.