I use timothy hay cubes as a supplement, as well as Weight Builder for my new mare (she came from a slaughter sale at 712 lbs. in January, and has gained 100 lbs. so far). I am sure that SmartPak also has something comparable. I do not think beet pulp has much nutritional value, compared to the other things that are available, but I’m not going to swear to that. What sort of grain is he getting? He might do better with a complete feed, and possibly more of it than you currently give, at least until he gains weight. Also I give a mineral supplement, because minerals are important and there is no way to figure how much of what he is getting from hay/grass/grain.
It is never the horse's fault