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.
I know a lot of people who run into this problem. It is really difficult - I think consistency is key. Firstly, it's important to be firm and clear in your expectations (and that should extend to all the adults in the family, not just mum) - at home, we speak Mandarin. I have friends that have gone so far as to completely ignore their child unless they use Mandarin. A bit harsh, in my opinion, but their kids grew up speaking perfect Mandarin - so well worth the effort. Another useful strategy might be to get him interested in Chinese cartoons. It's an easy and relatively pain free way to learn.
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thanks