think it may be a good idea to get some advice from a professional on this - sounds as though there is a bit of dominance involved here and unless you know what you are doing its not a good idea to try and cure the problem yourself. have worked with dogs all my life and learned very early on is that any sort of behavioural problem concerning dominance and/or aggression is best left to experts. I'm sure there will be other opinions but I've seen the results of folk taking on problems of this kind. what breed of dog is it, has he been neutered, how old and how long has he been doing this?