
To be honest, at the moment, I would only let him offlead if there are no other dogs around, unless you have excellent recall. He sounds like a typical teenager and very headstrong. He propably tried to mount the other dog to assert that he was the boss.
As he is big I can understand why the other dog walker was so upset ....it can be very scary I think for someone when a dog zooms out of nowhere and tries to mount your dog.
You have to take responsibility here as it was your dog that began the encounter rather than the other way around :)
Pretty sure more experienced people will be along soon to give more helpful advice than mine ...maybe more training?
Oh and I don't think castration will fix the problem to be honest....