I don't know about children, but I can speak from personal experience when I say that actively practicing another language is one of the best ways to learn it. I have heard that children are generally more able to pick up other languages however.
When I am able to attend school, I am a spanish major. I have found that practice to be especially true. It helps concrete another language in one's head that nothing else will do. There are basic teaching methods and groundwork practices to be upheld however.
If you know, what kind of learner is this boy? Visual, audio, hands on? Go from there when you go about teaching him English. It's all trial and error until you find something that works, afterall.
Good luck. I hope it goes well. :)