Dog aggressive towards other males

Athos was also mates with the Border Collie at the start, but hormones and temperaments change as they grow up! You’ll need to work on his "no" command and, as mentioned above, make sure every introduction is properly organised—almost like a ritual—so your dog understands that while he doesn't have to like every dog, his reaction should be to simply walk away rather than growling at them.

Thanks for your reply lewina! But even with a firm "no", he just won't listen. I don't get it though; some of the male dogs he’s going for used to be his mates, so I’m a bit lost...

Just to clarify, it goes perfectly fine with some males! Whether they’re neutered or not, it depends on a lot of factors. Sometimes Athos isn’t great at first and I have to tell him to be nice, but if the other dog is really chilled out (neutered or not), he calms Athos down and it’s all fine. Sometimes Athos clearly doesn't like them, but we’ll walk together for a bit without any issues because they both manage to ignore each other. Sometimes it just doesn't work at all and we simply go our separate ways. Other times, they hit it off straight away and no one growls. The important thing is that you can tell your dog to leave the other one alone if needed. And meetings with other dogs should always be "managed"—it’s not a case of "oh look, a dog, go and say hello!" No, it’s "oh, maybe a friend, stop, heel, sit. Hi there, is he friendly? Is he a boy?" Depending on the answer and the other dog's body language, we decide whether to let them meet or not, while telling our dog to behave himself.