Since you posted this in the homecooking forum, I'm assuming you're looking for some suggestions that go above commercial dog food? There is plenty that you could give him that would make healthy improvements to his diet
Since the basis of his diet is kibble, I'd suggest that any additions you make centre around meat (and offal, etc), but don't include much, if any, vegetable matter. That's simply because dogs don't really need any plant (incl. grain) matter in their diets, but kibble of all types has it in large doses. So there's nothing to be gained from adding any more to his diet, but rather quite a lot to be gained by replacing some of the kibble with meat.
You could consider adding things like some lightly cooked ground meat (lean beef, chicken, lamb, turkey, venison, pork - whatever is going), canned or fresh oily fish such as sardines or mackeral (not tuna), eggs (not more than a couple per week) and offal (organ meat). A simple way of going about this might be just to cook up a big stew/casserole that's mainly meat and offal and then freeze it in smallish portions (enough for 2-3 days). You then defrost as needed. It probably wouldn't be any more expensive than buying canned dog food, but is likely to be much better quality.
If you just want to add a tablespoon or two to his meals to make the kibble a bit more appetizing, then the above is about all you'd need to do. If, on the other hand, you want to feed a greater amount of homecooked food in his diet, then you'd need to take out a bit of his kibble to avoid overfeeding. In this latter case also, you'd need to add a calcium source to your stew - but that's very easy to do (just add 1 tsp of crushed egg shell for every 1lb or so of meat).
