There are two things I’d recommend:
One: Maybe he is getting pickier with his hay. Try some varities, such as Timothy hay, which my bunny enjoyed. I either gave him the Oxbow western Timothy hay (red bag) or I bought the 25 lb bale of Timothy Grass Hay at Tractor Supply Co., And that was cheaper so I would recommend buying a bale from TSC (if you have a TSC near you) if you ever want to buy hay in bulk. I did the math and it saved me $350 from the small Oxbow bags a year.
Two: Look at what kind of pellets he is receiving. At this point he shouldn’t be on adult pellets. They should be quality pellets without muesli type ingredients, as quality pellets will include nutrients essential to young rabbit’s growth. I recommend Oxbow Essentials Young Rabbit Food or Sherwood. Small Pet Select is another alternative. Go for quality over quantity too.
I also wanted to ask, could he have a parasite or an underlying medical condition? What was the last time he went to the vet for a checkup, and what did they say?
These are just my suggestions. Please seek an experienced veterinarian if you have medical needs / questions. Thank you!