Temporary car insurance for son with Australian licence visiting the UK

You will need to contact your current insurer to see if they'd be willing to add your son to your policy for the 3 weeks. Some car insurance companies will bluntly refuse but there are others which will do that at an extra cost.

If you are not able to add him onto your policy, then you can buy short term car insurance for him instead. I know for a fact that DayInsure have a short term car insurance policy for people with Australian licences, so you can get in touch with them (or get the quote online) to see how much you'd be paying to insure your son for the 3 weeks that he'll be visiting.

Let me know how it goes.