If you are aiming for trying to get to admit he is "doing something wrong with JD"... you may be wasting time, and positive energies.
Figure out what is REALLY important to you, and focus on that first.
Do you most want him to admit that he's doing something "wrong"? Or is it enough if he just agrees to stop it? In some situations, it is neccessary to get the other party to admit what they are doing is wrong, otherwise they wont actually STOP it... they'll just try to hide it. I dont know if your exH is in that posotion.
However, to my mind, the most important thing would be to get him to commit to you at the basic level of not dating other people. If you want to put it to him as "stop acting like his mother", that is one way to go, i suppose.
If the quick comparison to his mom works... then great! Otherwise, if you sense that he just doesnt want to admit doing anything wrong... you might want to drop that part of things.
Getting him to admit that he is 'dating' JD, will be a tough battle. You may want to avoid the negative energy involved in that battle.
IF he agrees that [whatever he is doing], you need him to stop doing it, for you to feel comfortable continuing to doing what YOU were doing with him.... then it may be unneccessary to argue about the rest of the issue.
If he repeats himself with the story of "we're not committed to each other, I didnt do anything wrong"... I think it is a waste of your energy to try to argue right or wrong with him. (its actually probably why he brings it up. to deflect you).
rather, just stick with your statement, that for you to feel comfortable being with him, you need him to stop doing it. With her, or anyone else.
My current status: june 2006. Wife ran out and filed D. Finalized Jan 11, 2010, after 12.5 years M. 3 wonderful sons caught in the middle