Sorry, I would tell him I didn't intend to entertain his girlfriend at Thanksgiving, and that if he wanted to spend it with her, I had othetr places the baby and I could go.
Seriously - time for some boundaries here, don't you think? Your H is out of his mind.
I know, I know - you want to go back to being that trusting woman who didn't feel threatened by H's friendships with other women. That was me too - thought I was too good for that jealousy stuff. Now I see that I just helped enable H's eventual affair by not spelling out clear boundaries of what was acceptable in the R.
Now - if you want to do this because you have an evil plan to make JD see how into you the Ex still is - fine. BUT - this could turn out really badly. Like - Ex imagining a threesome badly. Like Ex bending over backwards to deny you and he are anything other than friendly co-parents badly.
Would Ex be calm and cool about YOU showing up with a boyfriend? Heck no! So DON'T settle for less for yourself.
Just tell him - you know, I thought about it, and I'm not comfortable entertaining your girlfriend, so I made other plans for me and the baby. Then DON'T ANSWER YOUR PHONE.