There is a mom in the same shoes as you in EK She keeps on making conversation with her son in Cantonese, and the progress is amazing gradually. But don't expect it happens within weeks. Please DO IT from today, then tell us the changes after 4 months. Don't give up. Your child can master Cantonese, Mandarin, French, English with no trouble at all Congras.
My children were not born in Hong Kong but I have always insisted using Cantonese at home. They have no problem understanding me although they might have difficulties expressing themselves in Cantonese and in those cases, they would use English.
I guess, you need to make it into a habit or even house rule that he needs to speak to you in Cantonese.
Do not give up.