I agree with the other posters that this is a behavior issue, most probably. You could have him evaluated for ulcers/digestive problems. However, since he's more than willing to eat other foods than his dog food that doesn't seem as likely a culprit as it being behavior issues. Depending on his size, he might need to eat 2x per day (most dogs, unless they are really large, only eat 1x per day). If it's he's needing to more than 4 cups per day to maintain a healthy weight, then 2 feedings per day is better, but for our girls (65# retriever/hound mix, and 75# GSD mix), they get 3.5 cups of food once per day.
Feed him at the same time you're eating, and whatever he doesn't eat within 20 minutes, remove. Remove all temptations -- put the cat's food up where he can't possibly get to it (like the top of the fridge if need be! Cat food is higher in fat which is why dogs love it so much, more than their own food), invest in a bear-proof garbage can or be very diligent about making sure the garbage is out of the house and out of harm's way; get the kids on board and let them know that he needs to eat his own food, which is made specially for him, and that people food is strictly off limits and police it.
So that is all the "stick' part of it. Now for the carrot part. Institute a 'nothing in life is free' training regimen--this will help keep him stimulated and also help reinforce that in your house, you are the alpha and he is not. (Google "NILIF" & "dogs" or "nothing in life is free" and "dog"--it's really easy to do). Then teach him some games/play with him, run him in the backyard or take him to the dog park and let him run & play. Pick a kind of dog food he likes (or guess if you can't tell; something higher end will probably be more palatable, than, say generic kibble; on the other side, my friend swears her 3 Mastiffs won't eat their high dog food unless she mixes a cup of dry Pedigree (the cheapo, not actually really very good for them dog food). Anyway, pick a dry dog food he likes, then mix in a couple of tablespoons of the wet dog food. Dogs love that wet food, and it is incentive for him to eat the rest of the food.
I like the Kong toys, and the Nylabones (not the edible ones, but the "durable" ones) if he likes to chew, or the pressed rawhide if you prefer something not plastic for him to chew.
In some ways, he's a lot like a little kid (smart dogs are supposed to be about the same intelligence and ability to learn as a 4yo child, I've heard), but bigger and better able to muscle his way into stuff....
Good luck & keep your chin up--he can be trained, with patience and persistence!