My 5 year old jack russell is becoming snappy. He runs at people when they come to the house or when they are leaving. He has bitten two friends on the back of the calf when they have turned their back on him. He also bit a friend on the bum when she turned to walk out of the room, when before he was playing with her mo problem. In at my wits end. I don't want him to hurt anyone, I've had a dog trainer in and he didn't offer any advice that has helped. I want to be able to have friends over without having to keep him in his crate. What can I do?