Here are a couple of tricks you can use to find a lost kitty, especially one that's not used to being outside very much:

-> When you go out looking for him, don't just call for him, use sounds he might recognize, such as shaking a kitty-treat container, or an electrical can opener (sounds like tuna!) - whatever works to get his attention inside the house. By all means offer those treats up if you see him.

-> If he has an indoor litter box with any of his "business" left in it, try putting a little bit of that outside, especially near the doors and corners of the yard. Indoor kitties can easily get disoriented if they get outdoors, especially if they get chased or scared, and having that familiar scent out there to follow can sometimes help them find their way back.

We once had a strictly indoor kitty get outside, and we didn't see him at all for a whole week. But these little tricks (especially the litter one, I think) and a little bit of luck eventually led him home.

Hope your guy finds his way back, too!