Thank you so much for that very thoughtful set of replies! A lot of really great nutritional advice was given and it sounded like you were really on top of everything but there was just something nagging at me in his expression/appearance. I've seen it before when close bonded horses lose a mate and it just breaks my heart. They really can be lost and miserable. I'm not a horse but it sure looks as close to genuine grief as I can imagine.
It sounds like Drifter has gone through some tough times and a lot of changes. Some horses take it like no big deal and some feel it very acutely. I think it just messes with their sense of security and rightness with the world. Just like the rest of us no doubt.
I think your ideas about giving him back his job and "play dates" with his buddies is an excellent plan! He will miss his pals but he will also be able to see that they still exist in his world and if you can provide him a consistant routine he will bounce back quickly. For the horses I've been through this with consistency was the key. I wasn't able to bring their pals back but I did my best to refocus their anxious energy and provide them with a sense of safety and security. I spent as much time as I could just being there and actually "standing guard" for them while they ate or rested. You'd be amazed at how much comfort this can bring them. Poor guy's probably been feeling the stress of a being menu item for months now! A horse's brain sees the world as full of predators and he's likely been feeling completely exposed and alone.
He's really lucky to have you on his side! You clearly love him and are doing your very best for him. I bet he has a major turn around in no time.