t’s difficult to know why they get stressed-out. Sometimes you can’t figure it out, you just have to accept it as a fact and deal with it. Try giving him fresh greens rinsed in water so he gets fluids. Mint is often appreciated as well as other herbs (basil, tarragon, cilantro, parsley). Massage his tummy like for 15 minutes every 2 hours. Serve him small wads of hay throughout the day to increase the chance of him eating it. If he wants a craisin or a tiny tiny piece of milk-chocolate (the size of half of a little-finger-nail), it’s better he has that than nothing at all. Anything to awaken his appetite! If you have fresh pinapple you can try a little piece of that mushed up with pellets + water. Mush like that can be syringe-fed if need be.
I wish you the best of luck. My bunny Bam used to be a nervous boy with a tendency towards stomach-trouble. Luckily he’s been fine since May or so.