The register which lists Quarrier home emigrants dates I think from lmid to late 1880s - as children were dismissed from the home by their 16th birthday he is unlikely to be found in that list if his birthyear (I guess derived from a Canadian 1881 census) is 1866.

One common reason for legal adoption is marriage to the mother and thus allowing easier legal transfer of property to her children - I would look for that - possibly she had died whilst in some relationship to the adopter and he was providing some legal guardianship in this manner