I'm afraid i don't go for the whole "pack leader" school of thought.
Sounds to me like guarding behaviour, really common but can be difficult to train out. I've been in touch with a very highly regarded APDT dog trainer (who agrees with me re: pack leader stuff
) about my dogs guarding issues, and he's recommended clicker training, and using a click/reward every time he is nice to my other dog/doesn't guard. Tis as with everything, slow progress,
but we muddle along. I also have to make sure my dog doesn't have toys/treats around all the time to guard cos if they're there he's a nightmare, if they're not, he doesn't miss them.
If you have a look on the APDT or APBC sites you should be able to find someone really good locally.