how should I help my 4-year old son to learn Cantonese?

I have already tried a lot of things to help my son learning Cantonese, but he is still very reluctant to it..... He was born in NZ and we moved back to HK when he was 3. As my husband also doesn't speak Cantonese, so most of the time we have to speak English at home, unless I am with my kids alone, then I speak Cantonese with them. He always answers me in English, doesn't matter how much I've tried. I thought it would be better for him if his peers at school all speak Cantonese, but he is in an English speaking class of a local school, so the frequency of speaking Cantonese is still low. Shall I put him to a oridinary class in that local school next year, just to force him to learn? Or shall I find him some bilingual (Eng / Cantonese) summer activities to warm him up first? Any suggestions? Much appreciated!!!!

Hi, thanks for your encouragement. Daddy has been in HK over 10 years, and still doesn't speak Cantonese.... so I don't have much hope for the "boys" to learn the language together. My hubby is German, so he speaks German to the kids as well when he is with them alone, not sure if that's the reason why they are confused and have hesitation. Well, it's not easy....