I am terrified that he will do it again. I KNOW that they can contact each other should they choose. Short of locking him up, I cannot guarantee anything. He has her phone number memorized. If he wants to call her from another phone, he could.
The fact is, I feel a total lack of control because he is a person with free will. I can take all of the steps you propose and, lets face it, he can still do it.
This is entirely true. The only way for you to guarantee NC would be for you to be with him 24 hours per day for the rest of his life.
What you can do, though, instead of wringing your hands, is take all the steps that we tell you to take. The two things that you are evading or ignoring are the most significant things you could do to prevent a recurrence.
1. Expose to your son and your wider families. This will give you both support, and it will increase accountability. Your H will not want to hurt his son again.
2. Expose the affair properly at work. My suggestion, that you thanked me for, was not to tell the mailman, who is not in a position of authority and cannot discipline your H. My suggestion was for you to tell your H's line management and get their help to monitor his activities. Your H would be taking a big risk if he defied their authority.
If your H's shameful secret is known to his employers, to your son and to his parents, and if his employers monitor his workplace activity with a view to disciplinary action should he exploit their resources again, and if your H THEN goes out of his way to buy a secret pre-paid affair phone, or starts visiting internet cafes to send emails, or uses public telephone boxes to contact his whore, then you will need to separate from him.
However, if you leave open the easy means of communication that he has been using so far, and if you don't expose, you are not even trying to put a serious stop to this. You are just giving your H carte blanche to do as he pleases.
All of us would have an affair under certain circumstances (these vary for each of us). If it is easy to have an affair because it is easy to communicate and nobody can see, quite a lot of people will have one (60% of married people already do) and somebody who is already in an affair will simply continue it.
If it is hard to have an affair, those who go out of their way to screw over their spouses are seriously problematic and need to be separated from. You don't know which class your H is in yet, but start by making it hard for him. You'll probably stop the affair with the measure we have given you. You have a very high chance of doing so. You have a very low chance of doing so if you do not expose at to your son, your families and at work.