Show him the balls through
a red or green filter. Do not mention the filter's colour.
Edit: He might agree that there is some difference that he can't see, but he's still not convinced that it's the colour. Then
bring two filters, one red, one green (exactly the same colours as the balls), but otherwise identical. He may use both on the ballls. Switch them behind you back. Then he may use them on any reference object he likes, e.g. traffic lights.