The grandfather is Arve Tellefsen, a top-class violinist in Europe.
He was born in Norway in 1936 and is still performing at 85, including in the U.S.
His grandson is Joakim Robergshagen, born in 2000.
He was 12 when this performance took place.

https://www.youtube.com/watch?v=2keSKk8UUhM