Rebecca,
Since you are submissive to him, perhaps he would key into the dominant roll of a doctor "forcing" you to have an unpleasant medical treatment. Having an enema in a medical setting is perhaps one of the most submissive situations that exist.
You could tell him what a wonderful fantasy this would be and you would like to show him how to properly perform an enema (for safety purposes). If you can sell him on this, this is your opportunity to perform the most sensual enema experience you could conjure up.
I would keep the volume fairly low for comfort, and make sure there is pleasant anal and prostate stimulation as you lube him and insert the nozzle. For me, oral sex to orgasm during the enema would probably close the deal...
When I met my wife, I wasn't "into" enemas but she was. She started a conversation about all of the health benefits of colon cleansing, and finally convinced me to let her try it on me. I begrudging agreed. One night before dinner, she fed me a large Martini and then announced she was going to give me an enema. Gritting my teeth, I walked up the stairs to the bedroom and walked into a candlelit room with a bulging enema bag hanging from a hook over the bed. On a towel on the bed was a large nozzle, and a tube of lubricant. It was strangely erotic. She had me get in the knee-chest position and after stroking my balls and crack, began lubricating and massaging my anus until I almost came. Then came the nozzle and the warm water. While the water was going in, she reached between my legs with a slippery hand and worked me until I came. I still remember the feeling of the water just flooding into me after the orgasm and running to the toilet.
When I came out, she was grinning and asked "if everything came out okay?" She praised me for being so open-minded and doing such a good thing for my body. I was bewildered by all of the competing sensations - but I did enjoy it on a bunch of levels. She repeated this scenario on the following weekend and I was hooked. Later she taught me the ins and outs of the fine art of enema giving and we have been enjoying this now for over 15 years of marriage.
Hope this helps. Feel free to email/ backend@allmail.net
Len