Your best bet is to go to a large book store - fortunately they still have a love of reading and books in Korea - I recommend Kyobo Books -- there you will find a wide selection of materials. At that age it might be best to start him off with books designed to teach Korean kids Korean, because most of the books designed to teach Korean language to foreigners are aimed at adults who already have a grasp of grammatical concepts.
Of course placing him in a Korean preschool program a few hours a day with kids his age could be a great way for him to learn Korean, he'd probably be practically fluent in two years, but the culture shock might be a little harsh. I worried that our daughter's attending a Korean preschool here in the US might interfere with her obtaining a good foundation in English, but my concerns turned out to be unfounded. Anyway, she's in her second year at UCLA now