How awful for the mother. It would break her heart for him to be missing all those years and then to have him be so fully indoctrinated into the Dad’s life… More heartbreak.
This young man will need time to process all of this. He may come around and decide to re-establish a relationship with his mother, but that will need to be handled delicately. He cannot overcome in one week a lifetime of feelings that he experienced as a result of being told his mother rejected him.
I really feel for that mother. What hell to have lost your son, only to find him again and know that he doesn’t necessarily share your feelings of joy at the prospect of a reunion. Hopefully she can be patient and get some guidance from a professional on how to approach the situation.