Hi Jack's Mom!! And welcome to Pet Talk! I have never had a dog that wasn't a REAL chow hound that wolfs down her food(never refusing or not finishing every morsel) so I'm not sure I'm going to be too helpful, but, I think that maybe adding a few tasties to his food might perk up his appetite. You might try adding a little scrambled egg (not too often) cooked hamburger meat. some shredded low fat cheese, yogurt or cottage cheese or even some cooked rice. NOt all dogs do, but my dogs like all kinds of fruits and veggies. You could try some baby carrots, some apple slices, etc., but not too much....gas!

Has he always been this way since he was young? I know that some dogs (mostly smaller ones) are more "self-feeders" that like a bowl of dry food left down to graze at their leisure.

Has he always been on this same food? Did he like it before and just recently start to refuse it? He just might be bored with it! If you do decide to switch over to another brand, I would do it gradually, slowly over a 2 week period, encorporating the new food with his previous brand.

Not feeling well is of course something to consider. Any chance he may have a bug or is not feeling well? Is he losing weight? Did the vet have any ideas or suggestions? My Lab and Husky both eat about 1 cup twice daily with some veggie/fruit snacks and doggie cookies.
Hopefully some others will chime in! Hoping the best for your Jack!!