For the sake of your dog (and anything he comes in contact with) get some professional help fast! Find a trainer who understands JRTs, and be prepared for a battle!
Jacks are easily bored, prey oriented and possessive by nature. If they have had a hard life before you got them, they remember. If they accept you as the boss, they are wonderful, but for a small dog they can be as much trouble as a large angry animal.
My 11 month old puddin (female) has potential to be really "not nice" and I have been surprised by the battle of wills and ongoing effort it has been to train her that small and large animals, children and grown people are not prey to be hunted or enemies to be routed! The good news is, I am winning, and Boadie seems happier for being secure and knowing her boundaries.
