The best medicine is his own. Don't be available. You haven't been waiting for him while he goes off and does his thing (and even if you have, don't let him know!) so don't act as if you have. Giving him that second chance straight up will prove you are always going to be there no matter how long he takes off for... and he will do it again if there are no repercussions.
Why is he back? because he loves you? Can't get over you? You love the him too, right? You want to be with him? Ok -
Ultimatum time.
You have him right where you need him, to be able to do this. He's feeling vulnerable, he wants you back. Feel empowered. Tell him it's all or nothing. You're worth it, and he knows this or he wouldn't be back... he would've gone off and found himself another OW. If he says no - you've been without him for this long anyway!
Worked for me.