Should have waited for him to nut, then just as he shoots the first rope of cum, yell out "caught you jerking!!" and then dash out of the bathroom so he knows he was caught but doesn't know by whom lol. Then I'd wait outside the bathroom and secretly film his expression as he exits, desperately scanning around him in a vain attempt to identify the person who saw him at his most vulnerable moment hehehe!
But wow, it looks like he's got an impressive size stiff cock for the size of his body!