One of the pellet feeding charts that I know of, that was written by a former vet at the sanctuary where I volunteer, lists 1/2 cup daily for a rabbit that is 8-10 pounds, but that doesn’t mean that every rabbit of that weight should get the full amount daily. Maybe you could slightly cut back, by not filling the 1/4 cup measure to the very brim each time. Rabbits do get needed vitamins and nutrients from pellets, but they don’t have to have a lot of them. Many rabbits will start eating more hay when their pellet ration is cut back a bit.
Is there some reason that Mr. Buns is on alfalfa based pellets instead of timothy based pellets? Almost any rabbit would favor alfalfa pellets over hay. It’s good that you are no longer feeding the pellets with the dehydrated fruit, but plain timothy pellets are the best for an adult rabbit.
Rabbits can get gas from time to time, even if they aren’t getting too many pellets and eat a good amount of hay. He may have had that episode due to the switch in pellets, and maybe the alfalfa pellets are just too rich for him, but it’s not always possible to trace the source of a problem.