Let him read, read, read, read, read.
Books, magazines, graphic novels/non-fiction.
Take him to the library, take him to book shops.
Put on kid-friendly podcasts.
Take him to museums, take him on nature walks with relevant books and guides.
Take him to national trust properties and let him read each and every plaque and description.
Make sure he remains physically active as well as mentally active.
Let him be a child who can freely explore his interests in whatever direction he wants to take them in, in whatever sized chunks he wants, at whatever time he wants, however he's feeling.
There's plenty of time for tutoring (he doesn't need a one to one teacher at this age, that will almost certainly backfire).