A rabbit who is not eating needs to see a vet immediately as an emergency appointment. A variety of things can cause a rabbit not to eat. Without taking in any food a rabbit's gut will stop moving, this is called stasis and is very serious, without quick intervention it is fatal.
Rabbts are like people that when they do not feel well they just want to be left alone. So if your rabbit isnt in a playful and grunts at you, which female rabbits do, he might just getting old or is depressed.