Even though there is no legal issue in your son coming back and returning for study, there would the practical impediments, in particular if the airline cancels the flight and refuses to return the money spent to you.
The other possible practical hurdles could/would seem to be:
- caps on arrivals in Australia (it seems airlines manage this)
- paid mandatory quarantine (assume cost is no issue)
- approval to leave (showing study course should allow approval to leave)
- flight out (no caps to return to US)
- permitted entry on arrival (assume US don’t plan to impose a travel ban on arrivals in future)
- getting covid at any point (an airline would be within their presumed rights to not allow a positive passenger to board).