Is he wormed regularly? Even if so, have the vet do a fecal egg test, sometimes worms sneak past those dewormers ..
Have his teeth been done recently? Have a vet come out and look at them.
And, if all the above are clear, get him on a diet with plenty of fiber and fat. Especially fiber. I would recommend free choice grass hay, with a little alfalfa if you feed it, Beet pulp, lots of fiber, and rice bran, lots of fat.