Main question -- is he pooping? If so, what do they look like?
And, is he drinking?
When did he last eat? What were they feeding them at the shelter?
He definitely has to eat, yes, but he can't have an abrupt change fromhis previous diet. And right now, if it'sdifferent, he may not recognize his new diet as food.
Your best bets at the moment are to get some Nutri-Cal (it's made forcats and dogs but works great for rabbits) and squeeze a 1/2 inchribbon ontohis foot, mush it down and let him lick it off. And some Pedialyte from the drugstore (baby electrolytereplacement fluid) may help. If you can get him to taste itfrom a dropper or syringe, he should take it willingly.
You can also try Critical Care, which is something the shelter or thevet should have -- it's a powdered pellet and nutrient mix.
Or,soak his pellets in water for 10 or 15 minutes to make upa slurry mix and try syringe-feeing him that. If he doesn'ttake it willingly, try spiking it with just a bit of carrot juice orbanana.
Another thing to try is canned pumpkin.
Let us know how he's doing! If he's not pooping or drinking, he has a serious problem and has to get to the vet.
sas et al