We used to have a (much bigger) horse in training who who do this under saddle right after you'd get on. Quite a surprise at first! We broke him of it though, and now he's back being a productive citizen with his family.
I was going to say what waresbear said. I don't think he can stand on three legs.
You might try and support him some how when you work on his feet.
Maybe some type of A-frame contraption with some wide straps to run under his belly to help hold him up.
HAHA my shetland colt did something similar when we had the farrier out! He tried to LAY DOWN! Completely, like a dog! I was holding his weight up and finally the farrier said, just let him go! He dropped himself, but then he stood back up when he figured out we weren't trying to eat him lol