I think you have a lot more control over this situation than you realize.
Try to think this way....I need to really concentrate and do this too....He wants you. Ultimately, he wants you and his son and his family intact. For some reason, he's having a nutty. Is there anyway you could sit him down and ask, "What happened that we got here? That you think you need this other relationship? And please be honest, brutally honest so I know and we can work on this together. If you say it's because I didn't hug you enough, we can fix that. If you say it's because I didn't talk to you enough about whatever, then we can fix that. If you say I didn't ML to you every night after doing all the house work and cooking you dinner followed by a full body massage then cleaning your feet with my hair....well, we're going to have a problem because I can't provide that. So please, tell me what went wrong for you." And then you need to also put on the table what went wrong for you....even though you didn't go outside the marriage there's still probably some things BEFORE all of this that you would like to see changed too. Always room for improvement.
But it's time for you to act as if....act as if you know you are the prize, because we are the prize, not the OW. Act as if you know this is just a temporary thing that he had to go through to be able to come back to your marriage even stronger and that you two are going to make it.
If you talk to him again about your boundaries, I like what Puppy suggested and maybe tack on that you know it's going to be hard for him to cut it off, but you know he can do it, you are confident he can. I know it's helped in my relationship that I GAVE H some trust and made a big deal about it....that I knew he wouldn't disappoint me. (Jury is still out on this one, but so far, so good.)
You can do this Ashlee, and remember they aren't going to do crap in our time....or even the way we would. But he'll do it.
