The young king of Scotland came to Arthur's coronation feast at Wales with his 600 knights and other kings. However it was one of them who didn't acknowledge him and didn't accept his presents. (1,VIII)
He probably participated in the Battle of Carlion and the Battle of Bedegraine.
His identity is not certain. He isn't mentioned after his appearance in 1,viii, being one of the original 6 rebel kings. In 1,xii a full list of the 11 rebels (the original 6 and the new 5 ones) is given, but none of them is associated with Scotland. The best candidates are Carados or Brandegoris of Stranggore (if we suppose that "Stranggore" is related to Scotland) and, more tentatively, Clariance of Northumberland (if we suppose that Northumberland belongs to Scotland).
When Arthur ruled all Britain, the North had many kings who ruled in his name. (8,i)
Later there is mentioned King Anguish of Scotland, an ally of King Arthur. (5,ii)
Dame Lionesse announced a jousts between King Arthur and her knights on the Assumption of Our Lady so the King of Scotland and/or King Carados of Scotland (both title and name are mentioned), went to Castle Dangerous with Arthur. (7,xxvii)