This is the second time after a long period of R. I don't see why he deserves another chance, and I see no reason you have to wait. In your shoes, I would do some free consultations with lawyers to understand the likely outcome of the divorce (with respect to finances and custody) and then pick the attorney I like best and begin divorce proceedings. Right now, he's still full of guilt and playing nice. It's the best time to take action.
If you go this route, it's up to you whether you tell him about your meetings with the lawyers. Either way, tell a close friend/family member what's going on. You should definitely have someone in your corner to support your through this, and you might need school pick ups or childcare help, especially if he turns angry and/or walks out on you.
As for R, obviously that's your decision, but my feeling is that if you offer R to him again, he will have zero incentive not to repeat this in 5 or 10 years. People who are sorry but keep repeating the same bad behavior aren't regretful but not reformed.
I'm really sorry you find yourself in this situation. Once is bad enough. A second betrayal after years of R must be incredibly painful.
[This message edited by NoThanksForTheMemories at 5:20 AM, Wednesday, January 28th]