6 month old JRT acts aggressively to our Boxer/Mastiff mix!

by Lisa
(Texas)

Our Boxer mix, Flash, is a recently neutered, 6 y.o. male. Contrary to his name, he is not a ball of energy, at least not in the house. He's submissive and pretty obedient, excellent with my kids. He's very excited to meet new dogs, though.

Jack the JRT came to live with us two days ago. He's a pure bred, 6 months old, intact male. He lived in a home, mainly crate bound, because his owner worked and the owner's teenage son didn't play with him. He knew Sit, as well as how to sit to wait for his food bowl and leash. Now, at our home, he gets long walks twice a day, the run of the house at certain hours, daily training (treats and clicker) by my teen and myself, and plenty of play time with my younger kids. He's a feisty little fellow, a little dominate with the kids, but he's only six months old and hasn't learned the rules of our "pack" yet.

The problem is he is very aggressive with our older dog. We have so far kept them both crated, giving them alternating 2 hours of free time, keeping toys, bedding, and food dishes separated. We tried giving them a leashed intro yesterday. Flash was excited, but even turned away from him, waiting to be sniffed by Jack. Jack went and investigated, then slowly walked back to me. So Flash turned around to smell him, but Jack turned around and charged him, barking and snapping! Flash is confused, and Jack thinks he has won.

Am I supposed to continue to keep the dogs separated until we train Jack, then reintroduce? Maybe he just needs a week to readjust? I'm afraid if I wait too long, Jack will think his unfriendly behavior rid him of having to deal with Flash. I don't want to give him any ideas of him making our family's rules. There's no way I want to "let the dogs work it out" because I've seen small dogs before ignore pain to attack cats, coons, and other dogs. Does anyone have any ideas or experience in this???

Thanks!