Awww, poor baby! When he's a little bigger, you can actually burp him. Hold him with his tummy facing your palm and his little head and front paws between your thumb and forefinger, with his chin and paws resting on your hand. Rub and pat and rub and pat his back and he'll burp or potty or both.

Might be a good idea to put paper towels on your lap below him ...
After my experience with poor little Mortimer, I'm going to be rooting really hard for your bunny to make it and thrive! He probably can't eat lots at a sitting, so you'll have to offer formula every hour or two until you can get him on a schedule and he can hold enough to fill up his tummy. You can probably feel his tummy to see if it's nice and round after a meal so you know whether he had enough. When he's around four weeks old, you can start giving him soft food. I raised a couple of puppies who learned to eat baby cereal and canned puppy food at three weeks, and it helped keep them full. Watch his poop to make sure he doesn't get diarrhea. That means his food is too rich.
If I didn't have these two dogs, I'd love to have a bunny. They're so sweet.