What makes you think he is not being a kid? Just because he can handle advanced material doesn't mean that while off campus he isn't doing kid things. When did intellectually engaging with others with the same interests have a set age limit?
Or...Do you think he should be in high school where not only would he be held back intellectually, assigned mindless busywork, and subjected to the government school prison-like culture of protection gangs ( misnamed “cliques”).
My homeschoolers started college at the ages of 13, 12, and 13 with **one** course. The rest of the time, they studied subjects that interested them and did “kid things”. By the age of 16 they were taking a full load and two finished B.S. degrees in mathematics. Another child continued to study part time but excelled in a sport and excelled remarkably in this sport and in his studies. He now has a masters degree in accounting.
Now this approach strikes me as much more appropriate and not cruel at all. Taking a class here and a class there if the kid has an interest and an ability in certain subjects is a good thing. But this kid is taking a full course load.