I do not necessarily believe you need to accelerate his learning (as you state in your question). Little ones like this tend to do very well acquiring knowledge quickly all on their own! Offer him a variety of toys and books, as well as fun learning experiences -- just like you have been doing. A good book on this topic is Bringing Out the Best by Saunders and Espeland (Free Spirit Publishing). It focuses on gifted children under the age of seven years. Best wishes.
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