This question is difficult to answer. As far as I know, many insurance companies registered in Hong Kong are legal contracts and are protected by insurance laws and regulations of the Hong Kong Special Administrative Region as long as the contracts are concluded in Hong Kong. However, since you and your son live and work in the Mainland, both parties will encounter some troubles during the performance of the contract, which will lead to disputes and lawsuits. This is also normal, because the laws under the jurisdiction of the two places are different. You should be prepared for this. I think it is also a good choice for Chinese mainland citizens to buy commercial insurance in the HKSAR, but what kind of insurance you must choose, such as life-long pension insurance, you can choose to buy it in Hong Kong, which will not bring troubles, disputes and lawsuits to both parties when performing the contract. Like accident insurance, health insurance, or choosing to insure in the mainland, it brings a lot of convenience to the performance of the contract, and the legal basis for providing evidence is the same. I also think the following principles are very important: Hong Kong insurance companies cannot conduct business in the Mainland, and the contracts thus concluded are illegal. You should pay attention to this! You said "more than 20,000 RMB" in your question, so let me ask you, did you buy Prudential products in Hong Kong in RMB? This could be a trap!
Further reading: How to buy insurance, which is good, and teach you how to avoid these "pits" of insurance.