While living in Taiwan, I raised my oldest daughter entirely in French while the rest of the family (my wife is Chinese) spoke to her in Mandarin. She grew up fluent in three languages (she picked up English on her own because it is somewhat related to French).
As a linguist, I know that early childhood is a critical period for flawless language acquisition. I would strongly advise you to speak to your son entirely in your native language until it stabilizes. Chinese is very hard to pick up in an English-speaking environment. Multilingual Matters, a British publisher has several publications that can help you. I highly recommend reading books by Wang Xiaolei: Start with:
Growing up with Three Languages: Birth to Eleven
http://www.multilingual-matters.com/display.asp?K=9781847691064
That's a great book, never came across it but will take a look. It's true, Mandarin is difficult, but once it has been picked up, especially the writing bit, it helps in other languages too
Oh thank you with this great wonderful info @wentong-syhhae ! Appreciate it :)
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.