I am surprised no one has asked what the horses breed is or age what his/her work load is. These are important factors in trying to help with a weight issue.
a good grain when looking to help with weight issues and what I use is Natures essentials by Purina feeds AMPLIFY its great for weight gain and also for show conditioning skin and coat stallions etc. I use this product with my draft crosses.
I also found that Calf Mana 1 cup a day in feed works very well on TBs or hot horses.
Can feed all the hay you want but if the hay your buying doesnt have the needed nutrients its not going to help.
Check with your local Agricultural department to see about having your hay annalized for nutrients content.
Good luck TRR