Hello Ronnie Rosseland ,
Thank you for reaching out on the Microsoft Q&A.
As far as I'm aware, there is no age limit related to the MS Learn exams, however, I strongly suggest you contact Pearson VUE and ask about their policy. If they allow it, Just create a Microsoft Learn account for him, ideally linked to a school email. Then, check for student discounts here: Student Discounts. Once set, he can schedule the exam via the Certification Dashboard. Good luck!
I hope this helps! If you have any further questions, feel free to ask.
If the information is useful, please accept the answer and upvote it to assist other community members.