Honestly, game balance starts going out the window around 15th level anyway, so it really wouldn't do much, in my opinion, that couldn't already be done with other means. Your character is still limited to a certain number of actions per round, you just have more options to choose from.
And RAW is not off-topic because the point is what would it impact game balance. Since you can already duplicate the effect with other options, I'd consider it safe to say that doing it would be fine. He's still 3 levels behind in spell level access, so it's not until 20th level that 9th level spells are even an issue.
And RAW is not off-topic because the point is what would it impact game balance. Since you can already duplicate the effect with other options, I'd consider it safe to say that doing it would be fine. He's still 3 levels behind in spell level access, so it's not until 20th level that 9th level spells are even an issue.