Give him a tummy rub, gently. Here’s an instructive video: https://www.youtube.com/watch?v=JnuxGLa2reg
Do not do this if his tummy is very bloated, if it is and he’s showing signs of severe pain, rush him to the vet. (Obviously).

You can give baby gas drops to help gas-bubbles break up and to lubricate the contents of the intestines to get things moving. It’s bunny safe and can be bought in most places like Walmart etc. The substance-name is simethicone, but there are many different brand-names.

These are generally the first measures to take. The condition can, as you seem very aware of, become very serious, so you are right in taking him to a vet if repeated massages and baby gas-drops don’t get him back to eating and behaving normally.

Here is an article that may seem very long, but if you scroll down to the segments that fit your situation, it’s really good and helpful in these instances. (Nevermind the enema though, that’s more for pros). It’s by Dana Krempels who is THE major authority on house-bunnies: http://www.bio.miami.edu/hare/ileus.html

How is your bunny now ?

If he is not eating anything at all, give him Periactin. Its an appetite stimulant for children. If after this he’s eating by his own, its great. If not, you should syringe feed him using Critical Care or mashed wet pellets.

I am a little late to post. You got some good advice. I hope your bunny is feeling better!