i'm a bit confused about your actual question...so if i don't answer your question, i apologize.
based off the picture that you posted, i would change the feed around a bit. i don't know how tall your horse is, what the healthy weight should be. if that were my horse, i would start out feeding a high fat, high fiber complete feed. my vet recently put my extremely hard keeper on purina ultium competition. it is a really great feed, that you're not supposed to have to feed extra anything with. i took my horse off beet pulp (without molasses), and when i did that, he lost his top line. looking at your picture, i'm going to *assume* your horse would be very similar to mine.
also, you may want to consider feeding your horse more than once or twice a day. 3 - 5 mini meals per day would help to rebuild weight.
i would suggest starting your horse on the purina ultium competition by mixing in a pound or 2, per feeding, to the current feed that you are feeding. and adding 2 or 3 large handfuls of shredded beet pulp (soak it first) to each feeding. after a week of this, you can take a pound or 2 of your current feed away, and add in another pound or 2 of the ultium, while feeding the same amount of beet pulp.
i would not do any "real" exercise. hand walking is great. working on basic lead exercises is great. one of my absolute favorite hand walking exercises to help build muscle is to grab some ground poles (or whatever you can find that is similar) and placing them in an * (star) shape. then you just walk your horse around these, being sure to have the horse work both sides as equally as possible. simple, but very effective! you can also walk up and down some hills, to help your horse rebuild muscle. you can make an obstacle course to walk your horse through. again, not "big frills" stuff, but it's easy exercise for the body to help rebuild muscle, engages your horse's mind and helps you guys get more communication going.
just remember, when you exercise your horse, your horse is burning much needed calories. if you're just doing hand walking exercises, adding an additional 1/2 lb to 1 lb of feed should be enough to make up those lost calories.
if you'd like, i can see if i can find a link to share that could help you change up different leading exercises?
best of luck.