Have him pull an Austin Powers.
Er, well, I suppose not.
So far I haven't had an issue like that. I mean, really, I think most people get that it'd be pretty frigging stupid to pull out an ID like that unless it was real... I just feel really crappy when I get carded since I've had to ID myself 4 times in 4 days in the past week. Most ridiculous was when I was helping a friend shop for a sex toy for her birthday and we went into the media room.
You could, I suppose, acquire a carry letter. If you're that far into transition without one, well... you may have worse problems in the future.
Or, it'd be degrading, but you could try to pass it off as drag. Your safety may vary depending on your location.