The vet called back earlier. She said that the majority of the blood work turned out normal. However, he has a low protein and albumin levels. She said that indicates a malabsorption problem. It could be from a combination of resistant parasites and teeth. So she told me to do a Power Pack dewormer (and another dewormer that I can't remember the name of. I will call Monday morning to figure it out), then he will get his teeth done next week. She said to put him on alfalfa, she recommended pellets slowly introduced soaked with the rest of the grain in order to increase his protein intake. If that doesn't work it is possible he has an intestinal problem but she doubts that it is the issue. From what I understand cushings isn't completely ruled out as an underlying problem, but it is more important to get the teeth done, then test for cushings if he continues to have issues.
Hey I live int he same state as you, anywho lol. I have been following this thread interested to find out what was going on with him as I have rescued a few underweight horses. I am glad that you got the vet out and now are on your way to improving him. WAY to go.