When my dog eats tomatoes, it seems to relieve his bad breath. You can also get some doggie toothpaste or doggie chews. Be careful with the ingredients though. Make sure you check it and make sure there's nothing in it that they shouldn't have. I've seen it before, trust me.
Look in your dogs mouth and make sure they isn't any problems, like an abscess. If you don't see anything start brushing your dogs teeth. If it continues after brushing see you vet. A healthy dog should not have bad breath.