Look, it's really simple. Have him send you some XMR, then if he can trace it, he should be able to tell you.

1. when you've spent it

2. where you sent it

Of course, he will not be able to do that, since he purposefully (or even worse, unintendedly) uses ambiguous language to "prove" his point.

The whole point of ring signatures is that you can't tell who's doing the actual spending. Yes, there are statistical considerations to take in mind, no, it's not an issue (largely) except in a few well-known corner cases, and anyway, FCMP (soon™) will make it even less of an issue.