You need to mention how the two parents will have unique genes, and thus lead to further variation through sexual reproduction.
Also include that the resultant offspring will have a wildly different genetic sequence, leading to further variation when they themselves reproduce.
If he believes he is the only thing that exists, what else would he do with his time? Assuming that he assumes he is the only thing that exists, the only things he can do are to continue existing or to cease to exist.
Maiden has three guitarists, so you could try that, but then you would be lacking a bass player. You could try cycling through the bass during a jam and giving the others a feel for the instrument.