First read up on the subject. Don't forget to carefully explain to your inlaws what you're doing and why: it will make your son smarter and his thinking more flexible. Above all, don't treat your mother-son interactions as "teaching": kids (and many adults) hate to be "taught".
Just have fun in Chinese: simple cooking (kids love to help out), games, songs and play-dates with Chinese-speaking kids (esp. recent arrivals) etc.