I think you may want to politely explain and tell him EXACTLY what you are going to do. Then give him the option to choose someone else if he wishes.
If not you are cool, but either way at least you wont then get blamed, if the marriage collapses, for getting them off to a bad start by causing (perhaps in their eyes) an embarrassment at the wedding.
I've never been a best man. I was strange a kind of nearly bridesmaid once, wearing my tartan skirt, when I was about seven and my nanny Chrissy Macdonald got married, but other than that once I've never done anything like that. (I think I was probably supposed to be a Scottish page boy - but as I wore a long skirt and followed the bride down the aisle to me I was a bridesmaid.)