Welcome down under. There are over 56 beaches within greater Sydney, no other city has beaches close by.
We can suggest some beaches that would suit in Sydney if we know his skill level.
Manly and
Bondi both have board hire and are easily accessible , other than that we know very little about you to advise.
What other questions do you have about Australia?
Perth is a 5 hour flight from Sydney.
Melbourne,
Brisbane 2 hours, Adelaide don't bother going there for surf.
You may not realise Australia is as big as the continental USA or most of Europe London to Moscow And Norway to Greece.
If your son at 21 is a surfer he would have some ideas of where he would like to surf.
If neither of you have any surf skill or awareness then stick to the surf schools at Sydney beaches.
What country have you come from?

I've taken up troll hunting just for fun, instead of a rifle I'll just use a pun! 冲浪爷爷