I've been going through a simila thing with my old guy Rooster (see my thread about Rooster also in this section). When his weight dropped off, we started him on Empower, beet pulp and rice bran oil. The weight came back but surprisingly, he got a second childhood! He has been so revved up that he's been spooking at everything and whirling in mid-air!
So we've backed off on the Empower for now to see if I can get my old, serene, reliable fellow back.
Try the beet pulp, rice bran and Empower just the same. Just watch out that your horse doesn't get too excited or nervous or overweight with the diet change.