I think "Czy on jest jeszcze artystą?" would be "Is he still an artist?" - it's hard to show it on the example of an artist, so let's go with president: "Czy on jest jeszcze prezydentem?" = "Is he still the president?"
If I understand your English sentence correctly ('yet' was always a bit confusing to me), it should be "Czy on już jest artystą?" - it means that I ask if the moment when "he" could call himself an artist has already come.