Lots of really great points being made here.
Everyone is different, of course, and it seems important to me to remember that timing on any sensitive subject can make a great deal of difference.
My suggestion is to wait until the two of you are both very relaxed and have not had any recent arguments. After snuggling with him, tell him that you want to talk about something important to you that you are a little embarrassed about and get his assurance that he will listen, even if nothing else happens.
Then, tell him that you have always found enemas to relaxing and that they make you feel romantic and aroused (use the words that the two of you use when you talk about such things.)
After he says, OK, say something like, "I know this may sound weird to you, but I would really love it if you could help me take an enema: Can you do this for me?"
If he says OK, try to get him to give you a small one right then: be ready with everything before he can become uncomfortable.
Then praise him no matter how badly he does and find a way to thank him (you probably get the message here.)
Hope this helps!
John