Whenever I was feeling concerned about my son's relationship I sent him an anonymous letter.
Why did you send the letter to your son?
I want the best for my family, my son needs to reconsider his relationship.
If you have serious concerns about your son's relationship, it's best to be open and honest about your thoughts. Instead of sending an anonymous letter, you should tell your son about your concerns directly. Your son may disregard an anonymous letter and see it as someone that is just trying to sabotage his relationship.
I bet I can make my son think otherwise if I get the help of someone who he will trust.
Well then you should speak with him directly, and not use guile and lying to try tricking him into what you want. He'll respect you a lot more if you treat him kind and explain to him your feelings about his relationship.