Very workable, depending on how the transition from jester to knight was made. Jester could've been an informal word for it, he might have held some other position in the castle, like a steward who knew how to juggle. Blood relation's not out of the question.

Alonso was a lonely child after his father died, so I'm sure there were some willing to be a friend (and source of entertainment [and someone to help him sneak out]) in his time of need. Someone personally close to the king will make the reunion all the more gripping.

PM me if you have any specific ideas or want some of mine!