
Welcome to Kevin-world! Your dog is being a perfectly normal canine teenager. This phase lasts from about 8/9 months till about 18 months, and the easiest way to deal with it is to go back to basics and ride it out. Castration is unlikely to have any effect I'm afraid - surgery is no substitute for training.
You say he can't be bored. What mind-exercises do you do with him? Now's the time to start lots of training-games with him - thinking is very tiring. You say he goes swimming once a week and that's when he socialises with other dogs. Does he get to meet and interact with dogs the other 6 days? The more dogs he gets to meet (especially at a difficult time like this) the less interesting they become and he's more likely to take more notice of you. As a rule dogs find their own species far more interesting than an alien one (that's us!), and the less they see of them the more exciting they become.
So, can you meet up with a regular group of other dog-walkers to walk with? In the meantime don't let him completely off the lead, but rather have him on a long line (washing line or similar) so that he has the semblence of freedom but you still have control.