I've found that a lot of weight builders are just fillers with little nutritional quality. I've had great success with Fat Cat - which is a Vapco supplement that I get through EquiSupps.con (she donates to a horse rescue too and is cheaper than SmartPak!) and it's worked great b/c it has essential amino acids in it as well as several other ingredients that help healthy digestion and build muscle as oppsed to just being empty fillers to put on weight w/o quality. It was the ONLY thing - coupled with the high fat, moderate protein feed and the rice bran - that woud keep weight on my incredibly hard keeper TB eventer.
Good luck - he's a lovely horse and while not skinny, is a little on the lean side. This should help!