He has to want to learn. He has to be interested in things. Not all children are. If a child has no spontaneous love of books, stories, language, etc, or shows no curiosity in animals, nature, science, then you can't force it.
The first thing is to nurture a passion. In my experience, people generally lean towards either science or the arts. Expose him to as many different things as possible – take him to art galleries, plays, science museums, zoos. Take him hiking in Scotland, or fishing in Cornwall, and so on. When you do these things, observe his reactions. Don't make too much fuss, but watch and wait. If he's as bright as you say, something will click. Something will catch his imagination, like a tiny flame, and then woosh. My brother is doing a PhD in Shakespeare, and even today he puts his love of language down to Kipling's Jungle Book. My grandfather read it to him when he was six, and the language mesmerized him. I bet every great physicist, astronomer, poet, novelist, engineer, architect, or whatever, can trace their whole career to something similar.
Second, don't push him. Kids will resist if they feel they're being pressured. Some parents are just way too rigid and humourless. You need to create a certain atmosphere at home – happy, fun, full of laughter, but also full of books and conversation. Surround your child with beautiful and interesting things. Fill the house with DVD recordings of Tom Stoppard plays and Japanese cinema. Have art books laying around everywhere. Have rows and rows of the classics – Dickens, Woolf, Austen, Hardy, Tolstoy, – on the shelves, and let him find them.
Third, cultivate a sense of wonder. For example, when you go for a walk, look up at the stars and say to him, "god, isn't it amazing to think that there are 100 billion stars in our galaxy alone, and our galaxy is just one of billions of other galaxies, each with their own stars and planet. The universe must be full of aliens...I wonder what they look like. You ought to read H G Wells' War of the Worlds. Shall I read it to you?" Do the same with everything – with trees, animals, paintings. If you live in wonder, you'll pass that on.
Above all, read to him. Read, read, read. My happiest childhood memory is of being read to. In particular, read books by writers who love language, and whose language is vibrant and alive: Roald Dahl, P. G. Wodehouse, Lewis Carrol, The Wind in the Willows, Tolkien's Hobbit. And read some non-fiction as well. Richard Dawkins wrote a science book for children, called The Magic of Reality. In particular, expose him to books full of the 'wow factor' – books that contain amazing scientific facts. Marcus Chown is especially good at that. I'd also recommend a book of Richard Feynman quotes (it's called The Quotable Feynman).