Oh the bit about negative reinforcement wasn’t aimed at you, I know you’re not punishing him!

Yes, it definitely sounds like he has some issues around your dad being there doesn’t it! I think you’re making the right choice getting a behaviourist involved. And then if you can get the behaviour sorted, then go for surgical castration. I know what you mean in regards to avoiding health issues - I didn’t get my dog castrated till he was 6 as he had issues I needed to get on top of first but I was so nervous about cancer!
I’d definitely give that nurse a go! Hopefully she can get a plan in place for you and if she thinks it’s more complex she may refer you onto a clinical behaviourist who would come out to your house. Let us know how you get on, I find behaviour fascinating!