It could be caused from several things...I would think given his age, it may be due to his adult teeth coming in, as others have mentioned. Or possibly something with his diet. I would recommend making sure to brush his teeth (for more reasons that one). Also, they make drops you can add to drinking water that freshen breath and help breakdown plaque. I use it with mine and swear by it--even our vet commented on how clean their teeth are.
