Well, you mentioned that if you hadn't moved 800 miles from home 4 months ago you would leave immediately. If that is how you feel, I dont think you are doing yourself or your son any favors by trying to sacrifice more for her.

You left home, for her.
You gave up your job, for her.
You are a free babysitter, for her.
You are not confronting her, for her.
You are letting her ingratiate OM to your son, for her.
You are turning a blind eye and keeping her affair a secret, for her.

You have done enough for her. For everything you have done, she has responded by lying to you, cheating on you, betraying you, emotionally abusing you. And you are helping her do it. So why shouldn't she? There are obviously no consequences to her actions that hurt you and your son.

Different marriage support programs have different recommendations on whether to snoop, whether to expose affairs to family/bosses etc, whether to tell OMW, whether to do anything to force your wife to make a decision. This program is firmly in the no camp for all of these. In many cases I think this is the right track, but I really don't think this is best for your situation because you have put yourself in a terrible situation where you are dependent on your wayward wife. If she files for divorce in your current location, she can force that your son has to stay in the new location. You will be financially devastated, trapped in the new location away from your family and career prospects, and most likely will get bulldozed by her in the divorce and custody settlement.

Residency for divorce is typically established after 6 months. You are at 4 months in the new location. You can move back home, get your old job back and file for divorce in your original location. Filing for divorce does not mean you will have to get a divorce - if you file first you have control and you can drag it out or end it at any time. Your wife would be very hard pressed to get your son back to your current location, especially as she cant force you to move back and if you are not there, she would have to prove that she can care for him full time.

You are not taking her son away from her - you are giving her a choice: you and your son or her job and her affair. Which do you value more?