Angelina i.e. breaks to masturbate if you have good grades?
I feel like dedicated masturbation breaks, as well as "I will leave you alone without the device for fifteen minutes, wink wink nudge nudge" breaks would be kinda awkward in a parental keyholding dynamic. The parents knowing exactly when he is doing what, and judging it... even if you are openly talking about sexual matters, that seems like a step too far to me. If he gets such breaks, they shold be long enough that it does not feel like mom is waiting at the door waiting for him to... do the deed. And that goes for every break, be it regular maintenance or special events, but especially for any sort of "reward" breaks.
Kaja Masturbation is never a problem. So the sollution has be different.
He needs treatment like with an other addiction. Forced abstentions is not a solution.
And yeah, measures like forced chastity seem a bit dangerous as a form of addiction treatment, but for channeling a habit into something more productive, I see no issues with it.
But you are right that there are a few caveats to any form of chastity treatment for boys:
Regular ejaculations are considered healthy - it is generally not recommended to go below once per month over extended periods, especially at his age.
Unfulfilled arousal can also have a negative impact on cognitive performance, so it is unwise to commit to chastity periods that have not been tested in practice yet. If performance begins to suffer while the belt is securely in place, it is better to reconsider that arrangement than to double down on it.