I think it would be best to stay out of his drama until he actually makes some decisions that will stick. Can he dump OW and be on his own, or will he kinda dump her, go back to you, change his mind again, and a month and a half later you're going through it all again? Did they just have a fight and it's bounced him back your way? I'd be very wary if I were you and make him prove himself a little this time. Also, how can he explain away all the horrible things he's said? I'm not saying not to give him a chance, but you already gave him one easy chance, and this time he should work for it a little and firmly decide it's what he really wants.
I also think following through with mediating a separation agreement would be wise, just in case you need it.
