What sky said ^^ 24/7 hay is my fave.
Minerals, loose salt in a bucked. They will take what they need
A little high fat, preferably not sweet feed will help him/her along also.
A little alfalfa supplement if needed. Supposed to help with ulcers anyway. Not that he/she even has them.
True you shouldn't try to rush weight on. BUT that said, each horse gains weight at their own pace. In many cases when folks are taking months to put weight on your average body score 3 "thin" horse then the horse simply isn't being fed enough. (Assuming we are talking a feed issue, teeth, worms and other health concerns have been checked into) So yes, it is very possible to see a difference with the proper feed schedule in one months time.