Can you tell us what is in the hard feed he is being given including measurements? Then it might be easier to give advice as to what to add however you should build a feed regime up gradually so the horse has time to adjust.
Personally, I like Mitavite Xtra cool as it shouldn't heat him up too much. I think it is roughly 15% protein and you feed it with equal amounts of oaten and lucern chaff. I would aim to feed 3kg of Xtra Cool with 3kg of Oaten chaff and 3kg of lucerne chaff a day (split into two different feeds, morning and night). Xtra Cool is good as it contains many of the supplements that he will need such as magnesium, selenium, calcium, vitamins etc etc. Also it is steam extracted which makes it easier to digest.
Another good product is Cool Conditioner, it is oat free, protein content comes from soybean, copra and something else, I forget. You use less of this product, roughly 2kg CC-3kg oaten chaff-3kg lucerne chaff but it works well.
Another good trick is to add a handful of black sunflower seads to the feed, they are full of omega 3 fats so will help with weight and will make his coat very very shiny!
Having said this, it depends on the horse and what you intend to do with him so you may find that this is too much or not enough. Don't look for instant results, with the condition he is in I would guess it will take a few months before you really start to notice a difference.
Don't be disheartened! The gray horse in my avatar picture was in far far worse shape than yours when I got her. It took about four months before she had put on weight, built muscle and was ready for hard work.
Hope this helps!
All horses deserve, at least once in their lives, to be loved by a little girl.