If the letter to his victim's mother is only to be opened after his death, and she is seriously ill and may not get to read it, then it is a pity he has been kept alive all these years.
I am not a fan of capital punishment, but I would not go to much trouble to actively preserve the life of this specimen.
Let this not be a ruse as BBC suggests but he is such a sick b...... that you could never tell,I can't imagine the pain that the family must be going through,lets hope they find this child and put him to rest in the manner he deserves and give the family closure of some kind.