My first instinct is to tell you to check his incisors and jaw to to see if he has any teeth problems. Even if you’ve never noticed teeth problems before, it’s a good idea to check their alignment, as sometimes it gets worse with age, or it’s just minor enough that you will only see problems in time.
Rabbits can use their lips to shove back pellets to their molars, and greens are sometimes softer and easier to cut than hay.
Also, I don’t know about Canada, but I know this year’s U.S. crops of Timothy have been a little more brown. So you might want to try and just see if he’s not fond of the latest stock of Timothy. Try Orchard, Fescue, Brome, etc – mix ’em up even and see if that will entice him.
I would stay away from supplements, because many of them are too high in minerals and vitamins, and can cause a digestive issue over time.
You also said when you first got him he was a big hay eater, has his diet changed over time, I mean are you feeding him any differently? What does his diet consist of right now?