If it's a good deal for you, then I recommend going ahead and filing for divorce in order to get it all FIRMLY documented. Again, early generosity usually gives way pretty quickly to stinginess. And even if he signs a paper it really has no legal standing as you are not in a divorce. He could come back at any time in the future and claim that his situation has changed, he didn't realize how much bigger your pension was, he wants alimony, etc etc. WASs in crisis have a way of falling into financial ruin, and once that happens, all bets are out the window in so far as financial agreements go.
Also, make sure he's not counting on not having to pay child support. Child support is very important for your children's futures and you definitely should not let him off the hook for that unless he's going to be doing 50% of childcare and splitting all kid expenses 50%.
I know you don't want a divorce, but once he's said he's going to file and is living with another woman, you NEED the legal protection of a divorce (or legal separation - but in my state, at least, a legal separation would have been just as much work and e4xpenses, and then you'd have to pay that all again once you divorced). Plus, if he's offering you a good deal now, you should get it set in stone before he reneges on the deal (a VERY VERY common occurrence I'm afraid.)
