Start feeding him canned cat food. Make him want to stay near your house.
We have a barn cat named Ozzie that stayed at my stable for a year or two before he began wandering off to his "second family". It turned out that his new home was almost about half a mile away, and he was going back and fourth from there almost daily! We knew he would leave the barn and come back because he was sooo fat, but we found out where it was because the lady who was feeding him called us and (she even knew his name!) asked if she could put Frontline on him. Lol