Simethicone is ok for bunnies, I gave it to mine many times when he needed. Also, it's good to massage his tummy. I massaged my bunny's for a long time if needed, even put a mattress by his home and slept there close to him, to make sure he'd be ok. I always mixed a few drops of simethicone (bunny dosage) with a little bit of water, so it wouldn't taste too bad or be too weird for my bunny.
If you think he needs to poop, a vet here once told me it's good to give a spoon of olive oil. You can syringe it into his mouth (he won't like it). I don't know how good it is, but when my bunny was young (and I didn't know Rabbits Online) I did it. It didn't cause any harm as well, and my bunny pooped some time after having taken it - I just don't know if it was because of the olive oil or because he would have anyway.
Good luck there!!!!!!