How can you tell a dogs age?

Unfortunately its not an exact science, that guide is good, gives you a general idea, but the teeth will depend on what kind of food the dog is used to. If a dog is fed soft food, and not given anything to chew, its teeth can get dirty very quickly, making it appear older that it is, whereas a dog fed on dry kibble, or raw food, with lots of bones to crunch on and clean its teeth with, may have lovely pearly whites and look younger than it is.

Best of luck with it, I'd suggest taking him to the vets, let them have a guess:D