accept that he is going to see other people, lie about it to you and them, and continue your personal R with him for your own reasons. this means no drama when the OW stuff happens, you chose it, you own it. if you take this route, I suggest you make it clear to him that this goes both ways. no drama from him if you date OM and lie about it to him. It is an open R, he doesn't get open on his end and exclusive on your end.
OR
do not accept that treatment and cut off your personal R with him as soon as there are indications of it. if he tries to restore contact, then you can tell him the conditions under which you might consider it. you don't need to lay this out now, but only if it becomes appropriate. again, if i were you, this would include meeting with his C who he needs to be in therapy with, hearing him come clean with OW and ending it, and so on...
OR
do not accept that treatment and at the first sign of OW, choose to officially switch to an open R on both your parts.
Whatever. The thing NOT to do is to continue to be the loyal NM who is always there, still in a single-sided exclusive R, ready to be confidante and fck-buddy or lover, whatever SO wants, whenever the whim strikes him. That is the role that keeps the cycle going. Without you in that role, it stops, simple as that. Right now, you are the one spinning the wheel just as much as he is.
Also, quit worrying about what things sound like. It is better to do your best to communicate than to not communicate at all. If it is easier to express yourself in writing, then email him.
