I am currently working on my 20 year old paints conditioning and getting him to bulk up. For a while he dropped a lot of weight. When they are under weight I suggest not focusing on building muscle but getting the weight on. I dropped down to light rides 2-3 times a week and turnouts with lots and lots of food. Now that he is at a more ideal weight I am keeping him high on his food intake. (4 flakes of alfalfa, 3.5 cups of strategy, and 2 cups rice bran [per day]) On this he has been gaining weight steadily, and now I am working on muscle. He gets the same amount of food but now gets worked 5-6 days a week. Each ride contains long trots while moving well and holding up his back. Half of the rides include a trot up hill. While the other half includes transitions, spirals at all three gaits and other flat work. Then he jumps about once a week.
He is really building muscle and bulk everywhere, his topline, his chest/shoulders and hind. Good quality food, and enough of it really has to do with building muscle. Otherwise the horse that is already scrawny is just going to loose more weight. Other people probably have better exercises which I am subscribing for, but I just wanted to emphasize the feeding issue.