I find the Purina Equine Junior to be a great feed for young horses. Also to help keep weight on, I'd really recommend Equine SENIOR feeds. They are made so much to help the older guys keep weight on, they work magnificently for any horse. Purina and Triple Crown make really good senior feeds.
Corn oil, flax seed, and beet pulp (soaked) are all great, fairly inexpensive, ways to add weight.
Also, chopped hays (they come in "bales" in plastic bags) are GREAT. The horses love them and they do a really, really good job at keeping weight on. They are a little more on the pricey side, however.
Mineral blocks you can pick up at most local tack and feed stores. Your area really determines the minerals that your horse is going to lack, but the blocks to a pretty good job. The horses also like them and it's good for them in case they get bored.
Hope that helps you some.