Great. Thanks for the additional information. I really appreciate it.
Generally speaking, whenever a dog has a foul odor coming from their mouth it's one of two things:
Either dental issues...and although his mouth looks good to the naked eye, it's definitely possible that there's something brewing under the gumline where you can't see it.
Or, there could be an underlying medical condition...this can range from things like kidney problems to digestive issues...of course, without any other "real" symptoms such as vomiting, diarrhea, etc, it's not the first thing I'd think of.
There's also a possibility that it could be an issue with his anal glands. We most commonly see this if the dog has been doing any scooting of his butt on the carpeting. Their mouths smell bad because the dogs lick at their bottoms and the oily scent sticks to the fur on the face and in the mouth.
Since we're not exactly sure which is going on with your boy, the best option is to move up that PE appointment from spring to the near future. They'll check everything out and figure out which of these issues could be causing the odor.
I hope this helps.