Wow Bernie. Harsh but neccessary words. OP you have a major problem, especially being in Victoria. I have to ask if the dog is properly and legally registered ( Proper description) because if it is not then you stand a really high chance of having the Amstaff seized and euthed by the dog wardens. COuld it be that your partner has not registered the dog properly and is scared of just such an eventuallity?
If all is good and above board then a professional is the way to go. 99% of aggression is resultant from some fear the dog is carrying. It is essential to find what that fear is and what caused it so that you can start the long road to desensitizing him. Only a professional, working hands on with the dog, has the skill to find that trigger and work the dog away from it.
In the meantime all advice given above is good. I would also look up "Control Unleashed" and teach the dog the Look at That Game. Use his dinner, fed by hand, to show him that you can be rewarding. Make him work for each spoonfull. Sit, drop, shake a paw, spin, beg and so on.
YOu say you are able to get him to stay while you put his harness on. What sort of harness are you currently using? I have seen excellant results from the "Gentle Walker" harness and recommend it to all my students.
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