I’ll wait to see what he has to say, but nothing less than buckle end of the belt is warrented IMHO
Such punishments should never, ever be used for trivial reasons (as an extreme case the bizarre story about the man who beat his young boy for making fumbles in a ball game, getting in well deserved dutch with California for it) but if a kid, being raised in a family where he [or she, etc.] is taught that kindness should be one’s first choice, is then deliberately and voluntarily going out of his way to hurt people either physically or with a barrage of cruel words... well THEN the kid ought to feel some pain too. Then maybe he/she will get the sense that he ought not to be out there wishing pain on strangers gratuitously.