I need help with my 4 month old Chihuahua. I bought him from a friend who breeds chihuahuas when he was 8 weeks old. When I first brought him home, he was pretty scared. After a week or so he was finally comfortable in my home and was a normal, playful puppy. He would run and jump around the house and loved to play with his toys. But when he was around 10 weeks old, my other dog (Jack Russell Terrier) bit him after he went after her bone. It was a pretty painful bite that made his eye swell a bit and caused some more swelling under his jaw (chin?). He cried for about 2 minutes straight - well more like screamed until I could finally calm him down. It scared him real bad and he has never been the same since. Now he shakes all the time and always has his tail between his legs and his ears straight back. He always has this sad, scared look on his face and it breaks my heart. He doesn't want to play anymore (AT ALL) and only gets mildly happy when outside. If I'm not around he will just stay in the same spot shaking and not move at all. This has been going on for about a month or so and it just worries me that he will stay like this for the rest of his life. He hasn't made any improvement whatsoever even though I try to be loving with him.
I'm wondering what I should do. Will he grow out of this fear? Is there anything I can do to help him become how he used to be - a playful, loving chihuahua?

I want to help him with this problem while he is still young so it hopefully doesn't become permanent. Thanks in advance for any help/suggestions.