I had good results with beet pulp, soaked alfalfa cubes, rice bran (I only kept him on it for as long as the bag lasted, it really is expensive), and Step 8 (high fat pellets)
I also put about 1/4cup of vegetable oil on his grain.
You could try adding a Beet Pulp Mash to his grain
Beet pulp is good to help gain weight
Try ultium by Purina it’s super high fat. Also try their amplify weight gain
I use the Brooks fat'n fit or fat'n fibre whatever it's called it really gray!
I also hear that it helps to add some veggie oil to grain to increase the fat intake of your horse.
i used proadd ultimate. it can get pricey as well but 1 bag did wonders for my ottb. i dont know the sugar content but it is mainly protein