Has he recently lost weight or has he always been thin?
My last mare was always underweight, no matter what we tried. The vet tested her for everything and found nothing. I think she was just a lot older than we were told. She had worn her teeth down so bad from cribbing that it was hard to age her that way. When I went away for college, though, I sent her to a friend of a friend's farm to retire, where she passed away only a couple months later. Never found out why...the farm owner didn't consult a vet or anything. So maybe there was something wrong with her that the vet just couldn't figure out, or maybe it was from old age.
Sorry that doesn't have anything to do with your question. Just thought I'd share my "thin horse" story =P Definitely scrub down the stall, though!