If your dog won't eat what you're normally giving him just because he's bored, but the food itself is nutritionally balanced and stuff, have you tried mixing something else in with it? Ours get a stock cube mixed into their dinner, and any leftovers we have too. (They like gone-off cheese.)
My dog seems to eat everything. I like the Sainsbury's own range because it's not as solid as other dog foods. It tends to come out of the tin pretty easily and just falls apart so you don't need to mash it around to break up the jelly.
I don't spend a lot of money on my dog's food and I think a lot of the expensive ones would be too rich for him anyway. He has a very sensitive stomach.
My dog loves the Aldi branded dog food. I think it's called Earls. You can get a pack of smaller tins or seperate huge tins which usually get 2 or 3 feeds for my dog, though I expect maybe one and a half feeds for a lovely lab We also sometimes get her Hero meaty chunk tins, which are 12 for £5 at my local Asda. Also I always mix a tin with some dry food. You may do it already, if not then I suggest you do as it definitely fills up your dog much more. Plus I'm sure that just feeding them tinned food must get boring for them lol. Although we always stick to the cheapest brands we can get, they're always good quality to me. And my girl loves em (she's a very very greedy greyhound btw).