I know a TB who has similar issues. Not sure if when they sold him he got put in a bigger pen or not, but he was a HUGE TB, 18h and built like a WB, and I think his stall was too small, and being used to the track, not padded enough, and hate to say it, but owner was a cheapskate, and wouldn't get enough shavings for him, and claimed he had no problem, but EVERY morning without fail, I'd go out to feed, and I would watch him literally fall on his face. On the couple of occasions the owner did decide to buck up and get some shavings, he was a bit better, but I still wouldn't ride him or deal with him because it wasn't worth it. The vet did blood work and stuff and said that he was fine in that regard, so it had to be sleep related. I personally never saw that horse lie down, and I watch all the others, including my incredibly skittish Arab lie down and sleep at some point during the day/night. I hope there's just something maybe coming around preventing him from feeling safe enough to lie down, and that you can figure it out so your poor guy can get some good rest. I didn't see anything posted, and I know it's not your guy's issue, but apparently some horses can't actually lock their knees properly, and so often tend to fall as they are trying to sleep standing up. Thought that was quite interesting. Good luck, and keep us updated.