Can't say I'm convinced he would be/have been booed that much:
Sporting hero on the cusp of winning a record title (or at least winning more than other currently actve players).
Successful sports star v unpopular government.
A vaccine issue that has a sizable number of people on his side of the argument.
A sport that is constantly asking fans 'quiet please' so a fan base used to having to shut up.
A few early boos met with a dominant win making those booing concerned they're helping him.
The previously mentioned large serbian contingent there supporting him out of national pride
To be honest - some of those points don't stack up.