Budget really important to give advice here… Golf stuff goes from a few quid to hundreds.
Golf clubs are very personal, so unless he has been talking about a specific club he wants then you will get this wrong. So rule out buying him clubs.
I bought my dad some Titleist Pro V1 golf balls on his last birthday, they are basically the best balls on the market and he doesn’t normally splash out on them. He was made up. For a milestone bday though, you may consider them a bit disposable.
You can get golf balls personalised with his initials or somthing, ditto markers, pitch mark repairers.
Golf clothes if you know his size. If he is a regular player then he can probably never have too many golf shirts. Does he play through the winter? maybe some mittens to keep his hands warm between shots – you can get golf branded ones.
A 121 golf lesson (or lessons) with his club pro would make a good present.