In my opinion, it is very difficult to find someone, include legal practitioners, to answer your question.
Let me guess you are not a Canadian citizen, but can be a permanent resident with a Chinese passport. I understand the Chinese consulate will issue a Travel Permit for children whose parents are still Chinese citizens. If you are a Canadian citizen, I don't think your son is eligible for a Chinese Travel Permit. I may be wrong though since there are too many ways to deal with the Chinese consulate office.
Your son was born in Canada, so he is entitled to a Canadian passport. If so, he needs a visa and he can't stay in China for more than 90 days. I assume you want your son to return so that a relative, probably grandparents, can look after him for a few years, so that you can focus on your career. This has happened to lots of Chinese families.
Your son doesn't need anything to leave Canada. Put it this way, the Canadian government doesn't care anyone leaving the country.
The key is whether he can return. The Immigration and Refugee Protection Act and CBSA internal policies and procedures both indicate a returning citizen is free to enter the country as long as he or she can prove his/her citizenship.
In your son's case, the only proof is his birth certificate, if without a passport. The Chinese Travel Permit is not considered as a valid proof. If he stays in China for a few years and when he returns he probably cannot speak English, CBSA will question if your son is really your son. A lot of children are smuggled into Canada by another child's passport.
I have dealt with a similar case before. Though I can't share with you the details because of professionalism, I can assure you there will be big trouble when he comes back in a few years.
I don't recommend he leaves Canada without a Canadian passport.