The horse in the background looks a little underweight, too. can you take over feeding your horse? Even if you can't, if you go there every day, you could add a meal, keep it at home, and take a bucket with you to the barn.
Do you have Southern States Co-Op there? If you do, I recommend the Legends Performance, because it is high in fat. It has done wonders for my horses. It comes in "textured", which is grain, or "pelleted", so ask the vet if the grain is OK, or the pellet would be better. Just be sure to add it slowly, again, ask the vet his or her recommendation.
He looks adorable. My TWH, Sissy was a bit thin after winter, and she looks gorgeous now!
Good Luck, please let us know what the vet says!