Realitycrash said:
You might just be a different part of him, not a separate presence. No matter what question you ask or what truth you claim there is, he can just keep believing that this truth is something he already know (for you is he), or that it is something he is just making up right now.
You can suggest all you want, it isn't going to prove anything to him.
If he won't submit to logic, then proof is irrelevant. But proving my existence was what was asked for, so it isn't irrelevant.
Take my counting coins example from above. It is possible to have a dream where those exact events occur in the dream while one is asleep. So, that might not totally prove my existence, but it does prove a knowledge outside of his self--in the case of the dream it is subconscious reasoning, so the solipsist is force to admit that what he knows as his self is not alone--that there is at least one other thinking presence, which if I identify as "me", and give him my name, I cannot see why he ought not to accept it.
Any knowledge outside the self demonstrates a thinking presence outside the self.