His biggest need is fluids and to get his guts moving again. It is very serious. Hopefully he does not have a blockage on top of it all but grinding his teeth indicates he is in pain, possibly from gas. Get water or pedialyte into him however you can and try enticing him with some of the fruits and greens known to be safe such as fresh apples, plantain, cilantro or shepherd's purse. If he's eating applesauce let him but he might get better gut motility with slices of apple (skin on). I've hand fed apple and cilantro to rabbits that wouldn't touch anything else and it got their appetites going again. At this point it isn't a worry about spoiling his diet it is a worry about whether he will survive.