On a strictly practical note - even if you decide you don't want him back, wouldn't he be a better father and future co-parent if he got help for his problem? And it seems like, if you told him forget it, you're not taking him back - he wouldn't bother to try.
So why not encourage and support him through this stage - even if it means you have to fake an intention you don't really feel? You can always tell him later that it's just too much for you to get over, you thought you could but you can't. Meanwhile, him getting help and getting away from OW will help ensure that your kids don't end up with a stepmom who would send her own 2 year old away so she could have an affair with a married man!!!
