@tyco2003
That's explained, even if someone finds out about how his power works, as long as hes with someone who believes his power is cannon, he'll have cannon.
While it's not fully explained, I'm guessing the reason why he focuses on cannon so much is that it was the first ability that he was subject to. Also, just because he's invulnerable, doesn't mean he wouldn't feel pain, or be capable of dealing with someone, say like 'the ability to make swords that can cut anything', unstoppable force versus immovable wall.
It's also showing that he's still exploring his ability as well, he might not have had the idea as of yet, that if someone believes I'm invincible, then I'll be invincible.