3 year old dropping weight?

My foundation bred QH has always been a consistantly easy keeper. He has free choice access to Coastal round bales, gets one scoop of 12% pellets twice a day, and has access to pasture. He's up to date on all vaccines, rotating wormers, and floating teeth. He also gets sand clear once a month for a week, as I live in Florida. However, in the past month, he's been dropping a LOT of weight. His ribs are visible and the spine is protruding slightly. I had the vet out, and he said the horse is healthy, no worms or other illnesses. Has anyone had this happen to their horse? Would something as simple as beet pulp pick his weight back up even though nothing has changed in his diet since he was an easy keeper? I hate seeing him like this but it seems like no matter what I do, the weight keeps dropping. Any suggestions?