I would be concerned about his teeth, sore gums, or a mouth sore, if he has suddenly stopped eating the dry kibble and I would have that checked out first. To have him eat the mixture of kibble and topper, I would really mix the topper first with water, then mix in the kibble so that everything is moist. If the kibble is large that may not work. You can try to find a kibble that is smaller. I feed Solid Gold Barking at the Moon - that is a small kibble and it is grain free. For the calories - there should be some guidelines on the can. If not, I would contact the manufacturer.
Sting Chance von Gaard AKC GSD 2/8/2006