yes- i would get your rabbit to an emergency vet asap. once a bunny stops eating completely it is an emergency situation. It could be that his back molars are growing in jagged or pointed, causing them to poke into his gums when he eats, therefore causing him to stop eating altogether even though he is hungry
try mushing up some pellets with some applesauce and see if he will eat that. also- make sure he has hay and veggies to nibble on, as it is softer than hard pellets.
It could also be an intestinal blockage. Is your rabbit's stomach bloated at all? Is he in a shedding cycle? Also- if he free roams- did she eat anything around the house? All of these things can cause blockages that need to be pushed through her system. If it is a minor blockage your bunny might just need fluids and motility drugs to get him moving again. If it is a more serious block, you might need to force feed him and give him motility meds a couple times a day. Your vet will know what to do.
I have dealt with these problems with my rabbits before. A vet is the fastest way to get your bun back to normal. so, i would try and get your bunny to a vet today if possible. Or call first thing tomorrow morning and tell your regular vet that is an emergency and he NEEDS to be seen asap! If they won't take him in call somewhere else as this probelm cannot wait. You can try home remedies to clear up blockages, but if he has completely stopped eating you probably need actual meds and fluids. Also, if it is in fact his back molars you will need a vet as well.
Good luck with everything!
let us kow how things turn out!