All you have to say is:
"I will be happy to pass on your message and if he wants to contact you he will."
That way, you are not telling him where he is at, that is up to him. When and if you write your son, you can tell him his aunt called, and you will have fulfilled your obligation.
You do not have to give them any information, as that is up to your son .................... even though he may not have been acting like it, he is an adult .......................... his choice to contact or not.
J M H O
Love and hugs,