Experience with visa extension (Stockholm)

Hi- Does anyone have experience with applying for an extension to a tourist visa (beyond 90 days)? My son hoping to do private music studies in Stockholm in preparation for auditions.

How easy to get extension
How long did it take to schedule appointment with migration board
How long did it take for them to process your application
Did they require a Swedish citizen to "guarantee the applicant's upkeep" during their stay in Sweden, or was it good enough to be able to show adequate bank account balances?