Looking for the name of a Fantasy book where magic ability determines royalty

Question

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.

Answer 1

This sounds a bit like Codex Alera by Jim Butcher. Main character Tavi is without magic (called Furies in the series), is raised by a woman, Isana, and a simple servant, Fade, he is not quite mute but simple minded, and her brother, Bernard. As the story progresses he learns about his heritage, gains the respect of the people and learns his magic. He does battle in subtle ways with the High Lord Aquitaine, who is trying to usurp the kingdom. And help the entire country with several outside threats all at the same time.

Answer 2

Definitely Codex Alera though you've got details jumbled a bit. And having Furies, assistance of magical spirit things, is the norm for all the Aleran people with the nobles being better able to use multiple or more powerful Furies. There is stuff about the nobles, by law, having to have children to pass along their greater abilities but this is jumbled up by many others having strong power but untrained and not confident enough to use it. And there are lots of bastards with great fury power that aren't part of nobility because magic alone doesn't qualify anyone, there is still legitimate birth issue and regular monarchy type rules.
  That part about not having the ability to use furies, what sets Tavi apart, is what's neat about him, that he has to solve things and deal with situations without magical assistance, relying on his intelligence and understanding of science or how things work. The Alerans ancestors (lost in space/time Romans) didn't originally use Furies and people have forgotten how to do things without this magical assistance. He studies this old technology and because of his life without magic aid and this makes him more cunning and stronger when his powers do kick in.

  My biggest disappointment was that he does get really strong Fury power and it is because he is the (previously hidden) son of the imperial line thus negating the previous theme of the whole darn book! And once again, his birth must be accepted as "legitimate", his parents actually married, to inherit the throne. The books take all these stands against penalizing bastards and against slavery and that titles aren't important, general anti-discrimination principles, then betrays them all repeatedly when it's in the interest of Tavi, the main character.
"This social injustice is terrible."
"What's that? I get all the perks and privilege?"
"Guess it's not so bad, things take time to change you know. Where's my palace?"

  There is a well narrated audiobook series of Codex Alera and all the titles have "Fury" in them (if you want to review and enjoy audiobooks).