Not sure about asking him to take it up the ass, he might very well enjoy it and besides, making him uncomfortable isn't a positive thing at all. Be honest and straight and tell him that you are not comfortable with the idea. If he does not respect that, then you have a real problem. I think he will. good luck!

__________________
Lots of people talk and few of them know, soul of a woman was created below

I can't get through to her 'cause it doesn't permit
But I'm gonna give her everything I've got to give.

I hear your sweet voice calling
out my name
As I stare from a six foot cell
And from beyond I heard the words
Deceptively Yours