One thing I noticed was in the pictures of the other horses getting fed, there was a bay that also looked thin, you could see all his ribs pretty clearly, but due to the angle, I couldn't see if he was also really thin in the hind end. I think putting him in a stall to feed him is a great way to help him build up weight. The other horse is probably also low man on the totem pole, and thus lost weight as well. I did notice in the winter pictures, there was no hay around, which they really need especially when there is no pasture.
Your boy is definitely looking much better, I love how thick his bone is, and he's got a sweet expression. You've done good, and I can't wait to see more progress, as you work him more! Definitely keep the pictures coming!