If he's basically on stall rest cut his feed back to hay only. He doesn't need supplemental feeding if he has no where to burn it off. That may be why he reared. When my horse couldn't be ridden we worked on ground manners, fine tuning them. I bo't a $4 beach ball and threw it into the paddock and he went after it with murder on his mind. Sometimes I'd kick it away and other times toward him. Some horses enjoy a ball, others are pretty blase. Pick up his feet until he will lift with a vocal command. This leads to bowing which can lead to laying down.