Why does my kitten have bad breath?

I have a kitten that i saved back in august, He was being attacked by a dog. He is about 5 months old now id say. We have taking him to the vet a few times before because of damage the dog did to him. They fix him up and said he was healthy. We have been feeding him the same food from the start. And then a while ago started giving him some canned food every so often. We noticed a few months ago that his breath smelt kinda fishy, and now its even worse its such a nasty smell, before we could only smell it when he licked us, but now you can smell it if he is just sitting by you. He is a very very hyper kitty. He is an indoor kitty, he bounces off the walls. I also noticed that some times he seems like he is having a little trouble breathing, well it just sounds kinda raspy. Anyways i have looked up stuff on here and people say tarter build up and disease but it doesn't seem to fit any of those categories much. If you know anything let me know!!! Thank you!!