Beautiful horse! Just wanted to add (from experience) that when you're bringing a horse back up to weight, don't forget about supplementing with vitamins and minerals. When a horse loses muscle mass from weight loss (like yours did on the top line), the vitamins and minerals stored there can be depleted (like magnesium). What we feed daily has the content to maintain vitamins and minerals, but not replenish if that makes sense.
I've found that Red cell is a good supplement that is high in magnesium and other essentials. I don't add it forever...maybe use 2-3 containers worth. They're about $25 per container and a container lasts 4-6 weeks. You might want to ask your vet about that and see what he recommends.