Here are some resources for learning about equine nutrition.
Thehorse.com is a website with a lot of invaluable information. Under the Topics tab, they have a nutrition section. The Horse Nutrition Handbook
by Melyni Worth is an easy to understand primer on nutrition. It helps you figure out how many calories your horse needs and how many calories are in common feeds. It also does an excellent job explaining different nutrients and how the horse uses them.
FeedXL.com is a wonderful tool for formulating a diet, and they have newsletters about nutrition. You can also discuss particular concerns with a nutritionist.
There are many others out there, but these are my top three.
All that said, you do need to know what he's being fed currently, if he's being fed in a group, and his medical history. Once he puts on a bit of weight, the proper exercises are also important for helping him rebuild those atrophied muscles.