Im not experienced enough to know if he is dominant or nervous but my terrier was a bit the same, we now take him to agility lessons and he absolutly loves it, he has got a lot calmer and more confident. The borders there are brilliant, I think they are so intelligent they need something to think about. Perhaps you could go to obedience classes or puppy classes at the moment to establish that you are the pack leader, good luck, its great u saved him. we get a lot in rescue kennels in Wales as farmers will not keep them if they dont work. The cruel ones abandon them on the mounains to fend for themselves.