First Chinese visa for baby with British father, Chinese mother, UK passport

I am British, my wife is Chinese, we live in Finland and our son was born here in Finland. Since we applied for his UK passport he would now be considered British according to Chinese law.

Now we want to visit China with our son and will apply for his first Chinese visa. The Chinese embassy in Finland explains the required documents:

"Child with Chinese descent born either in China or in Finland applies for a Chinese visa for the first time, ... the original certificate for quitting Chinese nationality ... are required."

So although our son automatically lost his Chinese citizenship when we applied for his British passport, it seems there is still some paperwork required to prove that he is not Chinese.

What exactly is that certificate and how do we get one? Do Chinese embassies issue those?