If you only feed him one meal a day, I suggest splitting it and feeding him two or three smaller meals a day. This will help speed up his metabolism so he will burn calories quicker. I always feed my dogs two smaller meals a day rather than giving it to them all in one go now.

Make sure his food isn't too high in protein as well. 18% is high enough.

I also agree about a bit more exercise too.

Also, just because you don't know about others in the house feeding him sneaky treats, that doesn't mean it doesn't happen and it's surprising how little it seems to take to make them pile on the pounds so be sure to have strict talks with other humans who live with you about not feeding him surplus treats that are only going to be a hazard to his health.