Bunnies try to hide their sicknesses as best as possible until it’s really bad and they can’t hide it anymore. I think it might be the beginning stages of GI stasis, because it starts with a bunny less enthusiastic about food, and then smaller and smaller poops until there are no poops, and then that’s when it becomes very dangerous, which causes him to be hunched over and grinding his teeth. I think you should definitely visit a vet to find out what might be happening, and try to do it as soon as you can. He really needs his hay. This might just be the beginning of GI stasis. If he’s only eating his other foods and not hay it might be a tooth problem. Eating hay needs specific grinding and chewing motions, and those motions might cause him pain so he only eats his pellets and veggies. That would definitely call for a vet too. Try to not put out anything for him to eat but the freshest hay and water. Wait an hour at least. If he doesn’t touch it, give him other foods until you can see a vet just so that he doesn’t get hungry.