What a predicament! I certainly think that some specialised advice from one of the charities is the best first step. Stopping the boy from trying to meet his father will only make things worse, I think.
Strangely, my adopted daughter has never had the least inclination to meet her birth parents. They were both 17, and engaged when she was born. We know that their families refused to help them, and that they were forced to give her up for adoption. Attitudes were a lot different in the 60's.
Since we adopted her it has grieved me to think of her parents' loss. Did they go on to marry, and have more children? I'm sure that they have never forgotten her, as we were told that they were heart-broken at losing her.
It bothers me that they may have been waiting for 50+ years, hoping that she will contact them, but she has been adamant that she is not interested in finding them. I have carefully explained to her what things were like for unmarried mothers in those days, but she just says that we are her parents, and all that she needs.
It's good to know that she feels that we have given her all the love she needs, but oh, how sad I feel for her birth parents, and how happy I would be to reunite them.