My 15 year old son will be travelling from Vancouver to Hong Kong and then to Changsha.

You can apply for him a Chinese tourist visa L in Canada before leaving for HK. The invitation letter should include following information:
(1) Personal information of you son, such as name, gender,and date of birth.
(2) Purpose of the visit to China, travel itinerary such as date of entry and exit, places of visit, relationship between the inviter and the applicant, and who will bear the expenses of the applicant in China.
If he is invited by his friend in China, also provide copies of inviter's ID Card if Chinese; passport information page, valid visa or residence permit in China if foreigners.

I realize that I can get his visa in Canada before he leaves for Hong Kong, my question is more about if he will be able to do it himself because it is less expensive and we do not live anywhere close to a chinese embasy so the courier fee's etc would not be necessary. The biggest concern is if he can "sign for himself" being only 15 years old, or if he will require the signature and ID of his parent/guardian.

The application form for an under-age child should be signed by one of his parent or guardian and the relationship indicated. So, he may be not allowed to apply in HK by himself.