I'd start by dropping the message that he's on the "wrong side of this issue." It's clear that many people believe that's very much up for debate. Right or wrong though, he'll certainly anger one group or the other with his vocal position. That's the real issue.
If he took your position on the matter and was similarly vocal, would you have the same concern?
If he took your position on the matter and was similarly vocal, would you have the same concern?