No where in your letter do you say you are dealing with your son regarding this. Do you not find him a bit responsible for anything?
You write that your son has been subjected to this filth. By the tone of this letter, you actually make it appear that you your son is innocent in all of this.

I am in no way condoning any part of this service, but these services are out there by phone and online. And there is NO WAY to verify the persons age.
People lie about this all the time...and evidently your son is no different.

I doubt that you will hear from this company and if by chance you do they will most likely respond that they do have a disclaimer that states that this is an adult service and adult aged persons only are permitted to sign on.