he is living in Canada but not a canadian citizien so assume he does not have canadian passport yet.
so just use his travel document (assuming HKSAR passport) to go into china.
to my understanding. 回鄉証 could only be used if you are travelling from either HK/Macau to china, or else, a valid travel document is needed.
for canadian immigration - (for your own sick) you should use hte travel document where you have your landed paper - so that they have the date stamp to verifiy when you leave /enter canada....