What is the ruling on a son telling his retired father, "It is forbidden for me to take from your salary," knowing that he is unemployed and looks after his father's affairs?

If the questioner means that it is not permissible to benefit from any part of the father's salary because it is his private right, then this is correct in principle; for he is not permitted to dispose of his father's money except with his permission. However, if his father gifts him something, or permits him to use it, or spends on him from it, then there is no harm in that.