Regardless of his vaxed status or opinions if he had a clearance to come before his flight the problem rests with Australia and appears to be sour.
This country was one of a few if any which didn't even allow it's own nationals back blaming flights when really they were trying to slowly siphon them through quarantine. A national should never be refused entry for up to a year, if necessary they can quarantine in tents in the bush.
Anyway, if the Serb is allowed to play I'd love it if he basically fails to concede a set.
Australia has a right to deny non nationals like him for any reason but since he applied through the correct channels and was made to believe he was good to go it looks poor if he is sent home.