I agree with Peggysue in that he's getting entirely too much in the form of sugars/starch. My suggestion is to switch him to a complete pelleted feed at about 10 - 12% protein (depending on his exercise and energy level and work level - 10% is usually more than enough) with a higher fat content - around 8 - 10% is better than some feeds that average 5-6% for a horse that's a hard keeper. To help him hold weight you can add some stabilized rice bran pellets such as EquiJewel - as little as a cup a day can make a difference, or try muscle building supps that aren't just a weight building filler.
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!