Just to throw a different idea out there, he may be thinking that the only way to know whether or not he wants to keep working on your relationship is by trying out others. If that's what he's thinking, his interest in dating other women is not necessarily a sign that he's not interested in at least trying to fix your relationship. That said, I think the idea that in order to know he wants you he needs to try out others is misguided at best.
It's good that he agreed to talk to the new counselor about the idea. King's right: if you tell him not to date other women, either he's not going to 'obey' or he's going to resent you for it, which would be detrimental to the prospects of mending your relationship. I would hope the counselor will tell him that it's best not to date other women though, and it seems to me that he'll listen if he hears it from that source.
Don't let this revelation get you too far down. I'm not gonna say it's a good thing - obviously it's not - but a lot of people think the way he's probably thinking, that the only way he'll know he wants to stay with you is to know he doesn't want to leave you. It doesn't work like that though, and it's impossible to put forth the necessary energy to explore your issues if he's using that energy elsewhere. In short, try not to get too worked up before you've talked to the new counselor. I think you'll have a much better idea what to expect once that happens.
Hang in there
