Well,why do you think he doesn't generally do it with people outside his close family?
My observation is that ininstances of challenging behaviour,there are invariably individuals that the person with the CB doesn't go for.
It is useful to observe and consider why this is so.
With one individual,who has attacked almost everyone (including such acts as taking off his seatbelt,fighting off his escort,and ptting his hands over the eyes of the taxi driver taking him homw!) there is a peer who I have never seen him lay a finger on in the 12 years I have known these two.
Reason seems to be that this boy screams very loudly,the moment the other one comes within about 2 metres of him,even if the movement is entirely innocent.
This boy obviously doesn't like the screaming and agitated response (although from the sounds of it,your ds would.)
I knew someone else (grown man) when a support worker who would remove his clothes,defecate and smear whenever we tried to go out. There was another SW who he never diod this with however. She used to take him to the public golf course for an afternoon and there were never any problems.
In retrospect,her low quiet voice and dislike of idle chit chat (which I consider to be the trigger for a lot of CB in people with ASD-that constant stream of 'Hello! How are you? Let's just get in the car then...oooh lovley day and look there's Susan say hello.......etc etc')
seemed to have the desired calming effect.
Now,whether these individuals understood the Pragmatics of a social exchange is entirely debatable. They did/do however see that there is nothing to be gained from continuing with the CB that they had/have been displaying around other people.
I know another person who kicks and scratches a great deal in his educational setting but doesn't do it at home.
He lives on a farm and has more than three brothers so Iam sure you can imagine the response that hiskicking and scratching gets.