Once he's been to the vet to rule out anything medical, well, he's about at that "teenager figuring things out" stage in life. Keep a spray bottle of water handy, see what he reacts to - whether a loud hand clap or even hiss like an angry cat* - that will startle and interrupt him, and remember to be more stubborn than he is in these attempts.

*Paul does this really well, but I never have been able to. I would swear I am making the same noise as he is, but when he does it, cats run away, when I do it, they look at me in a milady curious way, but are not impressed, never mind scared off!