The first book begins with a boy who demonstrates no magic ability and is being raised by a woman and a mute servant. As the story develops, he ages and with age gains his magic talent late. His lack of magic helps him to develop his street smarts and a creative mind. He then finds out he is the grandson of the current king, who is very gifted in magic. His late developing magic gifts is shown to be the strongest and I think the current king dies and then the grandson has to battle to save the kingdom from another noble who is considered the next strongest after the former king.
Additional Info I remember:
The first time I read the book was probably over 20 years ago, it was part of a series of books.
I vaguely remember that the boy was the child of the King's son and a lady of a faraway or border kingdom, but the couple was secret about their relationship. The King's son had died fairly soon after the couple had fallen in love and the subsequent boy was not generally known to be of royal blood. But this stuff was only mentioned in the book after the boy discovers his magic abilities and royal blood.
The mute is also later determined to be a former champion of the Realm, like the right hand man of the King's son. One of the significant events in a later book in the series is the boy's (now grown up) ability to make a volcano erupt under his enemy's city, for which he feels remorse.