Approx. weight then? 4 or 5 lbs?
There are a few possibilities. He's getting too many pellets and filling up on those so isn't hungry for hay, his hay has mildewed from humidity and is no longer good for him to eat, you have a new batch of hay that doesn't taste very good, or he has developed dental problems and it hurts for him to chew hay.
I would check the hay, make sure it doesn't smell musty or sour, have white or black spots in it, or have a white dust. If you had started a new bag/bale of hay when he stopped eating or his current hay may be bad, buy a fresh batch(different batch number than the last) and see if that makes a difference.
If it's not the hay then check for signs of dental problems like drooling and a consistently wet chin(not just after drinking) especially during and after eating, exhibiting a strange chewing motion or strange head movements, dropping food from his mouth, drinking excessively, not eating cecotropes normally. If he's having any of these signs then a dental exam is necessary.
Medirabbit (dental problems in rabbits)
If he's about 5 lbs or less, then maybe he's just getting too many pellets in his diet for his size, and if so I would try reducing the pellets a bit, especially if he really is a little overweight. I would go down to 1/8 cup twice a day and try that, and keep a close eye on his eating of hay and his poops. You just don't want him to still not eat hay and have his gut slow down and his poops get really small, as this could risk GI stasis occurring. If his poops do start getting small then I would put his pellets back to normal for several days until back to normal, and then try a more gradual reduction like a tablespoon or 2 less each feeding and see how that goes. But if it's the hay or a dental issue causing the lack of hay eating, reducing pellets isn't going to help or make a difference.