I must be nuts

In a bit of a situation and I'm not sure what to do about it.   I'm divorced with teens.  The ex has very little contact with me or the kids and we are all fine with that.  He was always absent even when he was physically there anyway. 

I have a boyfriend who is also divorced, no kids.   His ex walked out on him the final time after having left him several times in the past.  Not to mention she was constantly looking for her "next" husband on Match.com and other dating sites even when they were still married.  She has been married 3-4 times.  

He was devastated when she left even though she treated him horribly, took him for granted, et cetera.  

He and I have been together now for a little over a year and a few months ago his ex started contacting him again.  Emails, text msgs, and now phone calls.  Always on his cell phone of course and I refuse to snoop in that so I am not even sure how often it is happening. 

She tells him about her sex life with her new man, plus the ones she is doing on the side.  Then her boyfriend dumped her and she called him to cry on his shoulder.  

I know he has some unresolved issues because of the way she treated him and he said he just wanted to see karma bite her in the butt and then he would be done.  Well... that was supposed to have happened a few weeks back and he said he was satisfied now that she got a taste of her own medicine.  However... she is still calling, texting, emailing.  

I told him that it hurts me when he talks to her so much but I understood his need to have closure and I would have to accept that.  Now she is treating him like he is her BFF and has to tell him all of this stuff.  

They had no kids, no reason to stay in contact.  I have kids with my ex and I avoid him like the plague.  

We had an agreement that we would NOT lie to each other for ANY reason, no matter how small a lie, given what we both came out of from our previous marriages.   HE brought this up and I totally agreed.  I have a feeling now tho that, even though he knows I'm hurt by his contact with her that he continues to do it and, instead now of telling me when she has contacted him, he is just not telling me about it.   The only reason I know is that he occasionally slips and mentions something she told him and if I ask him when she told him that.... he backs up and tries to cover. 

He says I am the nicest person he has ever known and I'm wondering if he is translating that to "you are the biggest sucker in the world to believe everything I say."

Dont want to start any fights with him but not sure how to approach this topic with him again without coming off as a total nag and completely insecure. 

Any suggestions?