You can try some Bene-Pac. It's a probiotic for animals. You can get something similar at TSC too, it's called something else, (I'm blanking) but it's the same thing.
If he's gassy you can try simethicone. It's in the baby section of a drug store or department store. It's drops that break up the gas bubbles.
Also if your rabbit likes to be petted or held, try gently rubbing his tummy.
Goold luck! If it gets worse, better safe than sorry and call the vet!
Edited to add: I just read in another post, the probiotic you can by at Tractor Supply Company is called ProBios. It's for horses and cows and is the same as Bene-Pac.