Once you've made all the financial moves, I suggest you calmly tell your wife. You don't want to appear to be "keeping secrets" like she is, and it's The Right Thing to Do.
Just say "considering some of the decisions you've been making recently, I thought it best to protect myself and our son." If she gets pissed (and she will), just validate and say "I understand you're angry right now. I did what I felt I needed to do in order to protect myself and our son, and as a courtesy, I felt you should know. I don't expect you to agree with me."
Assure her (and it would be a good idea to do it in writing, so that you have a copy) that you will continue to pay for the family's NEEDS, but that you will in no way financially enable her affair, and you're just trying to protect yourself.
If an expense is in doubt -- esp. if it involves your son -- err on the side of paying it. But I certainly wouldn't pay for her cellphone, for starters, nor anything else that she is directly using to run away from the marriage.
