Well I'm back, and things didn't go as planned but I think they ended for the better.
I had been set on going with FancyPants' suggestion and offering to show him in the urinal, but when push came to shove I couldn't even bring myself to suggest that. Instead I just told him it would really make me feel uncomfortable and he said 'no problem then'... just like that. I guess I had overreacted and been reading way too deep into it, coz there doesn't seem to be any change in our friendship and it was all business as usual afterwards. Sorry for wasting all your time here, though I do appreciate all your help (guess that's why this site has Support in the name).
On a side note, in response to some of the other posts, I can show my wang here because none of you know me and, more importantly, you aren't in close physical proximity to me, which is what I wanted to avoid and why I had no problem with sending him a pic.