2 yr old male Jack Russell showing aggression
Our Jack is 2 years old. He isn't neutered. He is the most pleasant, friendly dog you will ever meet - all he wants to do is play and cuddle and lick. He is absolutely brilliant with other dogs and plays with other dogs in the park all the time. He also has two female dogs - one Jack Russell who he gets on really well with and even stays with her when we're on holidays and they share a bed.
However, he does not like other male Jack Russells at all! Our friend has a 11 week old Jack pup who he brought to our house. The pup was stealing all of our dogs toys etc and our dog got really aggressive and territorial. He started to bare his teeth and also tried to bite the puppy on the ear. He just kept snarling at him every time he tried to go over to him or play with any of his toys.
Is this normal behavior with 2 male Jack Russells? Please don't say we should neuter him because we have already researched that and it is not an option. As I said, he is so friendly and pleasant that everyone just falls in love with him when they meet him.
Any advice would be really appreciated