I would try to stick with the feeds he eats. My old TB Loves a+m crumbles , soft to chew, and a sr feed that is produced north of me.
You could try the beet pulp, soaked, but add karo syrup or molasses . The higher fat is great for these old horses. The sugars may not be good for them, but look at this way, if he is eating it, gaining weight being happy, what matters more, another year or two , starving, or being fat and happy for a year ? I feed my old horse what he will eat , he has gained weight.
He is happier. I give him wheat bran, which he loves, and when it is Hot over 100, and I don't see the salt lick being used, I add a small amount of electrolytes, you can purchase them in the 'powder ' form. I also WET all of his feed. Put apple cider vinager on Siders food, he will pitch a fit.. he hates it. Applesauce, ground apples, shredded cooked carrots ( have to be soft) . I am going to see if will eat some watermelon this year. He has in the past.