Just because he's tall doesn't mean he isn't pure. If he goes over 15 inches you won't be able to register him though.
Next time you speak to the breeder, ask her how tall the parents are. He probably inherited his height from them.
Just because a dog has horrible conformation, or is tall, or short.. doesn't mean they aren't/can't be pure. It could mean they aren't pure, or it could also mean they inherited bad traits from their parents.
All of our Jrts are with in the registerable height limit. However, some people like bringing their unregisterable terriers anyways to compete in agility and other things. I once met a full blood Jack that was a total of 17 inches tall.
However, I see where you put he was black and tan. No white? At all? The standard requires for your terrier to be at least 51% white to be registerable. And if he has no white at all.. theres a high chance that he could be a mix. Because quality or not, most Jrts do have white somewhere on them.
Hope that helps! Have a great day!
Check out the standard and breed club at:
www.jrtca.com