You can do a gentle tummy massage like this, but if his belly is hard and distended, don’t:
https://www.youtube.com/watch?v=JnuxGLa2reg
If you have baby gas drops, you can give 1-2 ml of those once per hour for 3 hours, then 1 ml every 3-4 hs, they’re bunny safe and may help break up big gas-bubbles.
If your vet is rabbit-savvy, he/she should look at and palpate his tummy to feel for hard lumps or gas. My vet tend to do x-rays to check for obstruction but I don’t know if this is standard procedure everywhere. The bunny’s temp should be taken (it’s often low when there’s tummy-trouble). Then they should try to examine the teeth, both the incisors and the molars to see if there are molar spurs, a common reason for tummy trouble. They should also ask about his diet so they make sure he’s getting a healthy diet with lots of fiber.
Fluids are often given to soften up any lumps of food in the intestines and to perk the bunny up a bit, since if he hasn’t been drinking, that in itself can make him low. Sometimes vitamin B is added to the fluids.
Meds that are often prescribed are reglan (reduces amount of acid in the tummy), cisapride (stimulate movement of gut) metacam (tummy trouble is painful and the pain itself may make the bunny unwilling to eat. It does however further lower body temp, so it’s extremely important to keep the bunny warm). If the bunny tummy is empty, you may be told to syringe-feed him Critical Care (or an equivalent) several times a day. Your vet may not prescribe all these meds or none of them. It depends on what the examination tells them and on their judgement.
((((((((Yeti)))))))