Maybe try a couple things to see what he likes. Maybe I've got weirdos, but my dogs have never really like the original Kongs. I think they are too difficult. Wiki will lick off the "easy" stuff, but gives up trying to get anything too far down in the hole. She loves this Kong bone though - https://www.chewy.com/kong-quest-bone-dog-toy-color-varies/dp/51555. And also those bones you can get with filling then refill, something like this - https://www.chewy.com/barkworthies-shin-bone-stuffed/dp/171746. None of these things will last all day though, they really just take minutes for the dog to finish. You say you are leaving him for the day, have you left him for shorter periods at all?