What is the ruling on repeating the word "haram" when a husband beats his son?

The default is the prohibition of uttering the phrase "it is forbidden for you" (haram 'alayk) concerning something whose prohibition has not been established by Islamic law, due to the Almighty's saying: "And do not say, concerning that which your tongues falsely describe, 'This is lawful and this is unlawful,' to invent a lie against Allah." (16:116) This word may sometimes be used to deter from something that is said to be forbidden, meaning "it is not befitting for you," and not with the intention of a legal prohibition. If this is the intention, there may be no harm in it, but it is better to avoid this utterance as much as possible.