At his age he is replacing some temporary teeth, especially his canines and molars. If you can take a good look in his mouth, you are looking for either retained temporary teeth -what you would see is a large tooth and a smaller tooth occupying the same socket, or open sockets from the shedding of the temporary tooth. Either of these situations allow for bacterial growth and odor. If you spot this situation watch close and give it a couple weeks to see if the retained temporary tooth is shed or if the larger permanent tooth erupts. If you do not see any of this or any sign of inflammation of his gums you might mention it to your vet next time you are in. Also, you can try an oral rinse like C.E.T. or even their soft chews to reduce bacterial growth and odor. Let me know if you have further questions.