My son is 3.5yrs old and he doesn't like to speak Mandarin at home. Even when I speak in Mandarin to him, he will reply in English. How do I get him to speak more? He speaks Mandarin to his Chinese teacher in school.

Since, the language is difficult, perhaps, he takes it as a school work, and wants to speak it when it is absolutely necessary, which is, in school. I suggest, do not force him to speak, as once, he will know you are forcing him to learn it, he will despise it more. I think your way of speaking to him in Mandarin is a good way to encourage him to speak the language. If you will keep on doing this, eventually, he will start talking in Mandarin. If not sooner, than later, he will. And by listening to you, he will become familiar with words and phrases in Mandarin. Also, you try putting cartoons that he likes the most in Mandarin.