The treatment ended. He is sound, happy and energetic. It is great to see him play with other horses and canter in the field joyfully. He also performs much better, no toe dragging... and he seems to track up better. Now, I have to concentrate on building his muscles :)
I just read through all these posts. You got some very good advice and were willing to follow it. Your horse looks wonderful again and I'm so happy to see that. Now, about that first vet (who in my opinion has no business practicing) I would go to the clinic and have a few words with him! I I would show him the before and after pictures and ask him to explain why he didn't tell you your horse was starving to death. And why he said your horse was lame when his toe dragging was due to his lack of muscles. Maybe he could learn something. I doubt it but I feel he should be held accountable. And the barn owner is at fault too. I love the fact that your horse is on pasture 24/7 but the low man gets chased off the food. It sounds like you can visit daily so when winter rolls around hopefully you can be there to make sure he doesn't get too thin again.
I have never had a horse lose weight even after long bitterly cold winters. There is no way a horse should lose weight unless the amount of hay isn't keeping up with it's requirements. Your horse wasn't starving, a bit thin but that is not necessarily unhealthy. Being tubby is worse.
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!