What should this no contact letter say ? The OM is a 22 year old idiot who will take it as a joke
The rules are pretty strict around here about posting other material, but if you Google "no contact letter infidelity" there are a lot of good resources on the web for examples.
It sounds like OM doesn't currently want contact and if he did, he will do it regardless of any letter.... and your W will do what she wants.
I think you need to insist that if your W wants to reconcile you will need to handle this as a "family unit." You must be included and part of any correspondence and interaction with OM (or a 3rd party dealing with the situation). This means that you will need to be fair and tactful in handling any situations with OM. Since his child is part of the equation and would be part of your household, you will need to be handling this as fair, reasonable, and adult-like as possible... in spite of OM's immaturity.
There is no arriving, ever. It is all a continual becoming.