I don't think he's any kind of terrier. I know I could be wrong. His nature is just too sweet to be any kind of dog with an extremely high prey drive. He LOVES my guinea pigs and is very gentle with them. And of course, you can see how patient he is with the cat.
The thing is, is that at his age he's already 60 lbs and growing every day, so I'd be surprised if he didn't have some kind of larger dog somewhere in the mix there.
An interesting thing about him is that he has both rear dew claws, both securely affixed to bone. Aren't there only certain breeds that have rear dew claws? I haven't gotten a clear answer to this question.