As long as he is getting enough physical activity and social interaction and is not unhappy, I am not sure there is a problem.
Or if there is one, it is that you, and quite possibly he, seem to regard learning as a finite quantity like a cake: one you eat it there is nothing left.
Learning isn't like that: it is infinite and some people are just wired to want to do it all the time, like people who are very musical often can't stop humming under their breath or very athletic people often struggle to sit still.
There is not school in the world that would even attempt to teach him everything he is capable of learning. Once he gets to university he may well end up writing dissertations about a subject that by the end he will know better than his tutor and that is fine.
First of all, I would try to find out why he is spending all his time working ahead of the curriculum.
Is it because he worries that he will not be good enough- in that case, he needs reassuring rather than encouraging.
Or is it because it has genuinely not occurred to him that it would be a better employment of his time to spend any excess time learning new things outside of the curriculum, things which may not come up in the exam in years to come but will enrich his understanding and flex his intellectual muscles and enable him to gain a deeper understanding of the curriculum?
I spent very little of my time as a teenager out with friends in the evening- instead, I learnt 2 foreign languages that were not on the school curriculum, read widely about historical periods which were not taught at school, and immersed myself in literature from different countries. Those have immeasurably useful in my career; otoh I can't say I have ever missed the joints I didn't smoke or the beer I didn't drink.
As an academic teaching in the UK I would say one of the most damaging attitude in education is the one that "real learning" is only what is on the curriculum and that once you have mastered that your only answer is going to be to go to university early because you have exhausted all the learning suitable to your age. No, you haven't. You can learn another language or another instrument or set up a chemistry or biology experiment in the kitchen or observe the mating behaviour in the local wildlife or read up on the Civil War or write a concerto or learn how to paint in oils or invent a new computer language. You will never exhaust learning.