Yes, he is very thin. His ribs, topline, and hips are protruding. He needs to gain probably another hundred pounds. Also oats have pretty much no nutritional value. Ask your vet. I would probably put him on a high fat feed or a senior feed for him to gain weight because he won't fill out very well on oat's. Purhaps try rice bran, some horses like some don't. I speak from experience, I used to feed my arabian horse sweet feed and oats a loooong time ago. He wouldn't ever fill out so I took him to the vet and she told me my mistake was his diet.