
One thing leaps out at me here, for three reasons.
> it’s been like this since 8 weeks.
This means that it is extremely unlikely to be a hormonal issue, so the implant (or indeed castration) is unlikely to help.
It also suggests that this could simply be how he is. Some dogs are born not being good with other dogs - I have one here - and the most productive thing you can do is accept that, not try to change it. Sometimes the trying can just make things worse.
However, it could also mean that there is something going on medically that he was born with, that has been overlooked, and could make a difference. It brings to mind my oldest dog, who I adopted at 13 months, and who had always bullied dogs prior to that. I was never able to make progress with it until at 6 years old, an MRI looking for something else found that she had narrowed nerve root canals in her lumbar spine. Congenital defect. She had been in pain since birth, and it was expressed in her behaviour towards other dogs. There were no other physical signs whatsoever. When she was put on painkillers, the bullying stopped immediately.
So with that last bit in mind, I strongly recommend you post about him on here, if you're on FB (I recommend joining if you aren't, just for this):
https://www.facebook.com/groups/332134427492077That is the ACE Connections group, the FB front for TTouch based learning run by Sarah Fisher (ACE being Animal Centred Learning). It's an amazing group and ACE looks at the whole dog, not just the behaviour: gait, diet, coat changes indicating possible underlying issues, every inch is considered. Recently there was a case of a dog whose only indication of anything being wrong was a small zigzag pattern in her down near the base of her spine. She too had a spinal problem that was causing her pain, but there was no physical sign of it otherwise.