At age 5,5, teeth issues is the first I come to think of. As a rabbit ages the bones in his face become weaker/thinner and this can cause slight malocclusion and molar spurs can form. (The thinning/weakening of skeletal bones happens to humans as well as we age, but we don’t have continuously erupting teeth so we can’t get molar spurs). Molar spurs can cause pain when the bunny eats. Some buns will still eat hay but be reluctant about pellets and veg, some buns stop eating hay.
So I’d have his teeth checked by a vet. The bunny as a rule have to be under GA for a proper thorough dental exam, but if the spurs are obvious a rabbit vet can sometimes see them with an otoscope. A good bunny vet can also check how the teeth in the lower resp upper jaw move against each other. Rabbits’ jaws are very sideways mobile, but the hinge joint can’t open as wide as ours can “horizontally” and the molars are far back in the mouth, so they can’t be seen without an otoscope.
I think you should start weighing him as well. You can weigh him in a bowl on kitchen scales once a week and write down the result, (if you’re anything like me – I always think I’ll remember, but I now know I don’t.) That way you can keep close track of his weight, it’s not easy to see subtle-ish fluctuations of weight with your eyes only.