This is slightly strange.
William is listed as a "boarder" with the "Masson" family at Burnside Croft, Newhills in the 1881 census.
By 1891 the "Masson" family have moved to, and he is missing from the East Achronie census list, which would suggest that he was not "adopted" by the "Massons", but just mistakenly listed as a "Masson" by the 1881 census recorder. He then may have found out that he was illegitimate, and adopted the "Masson" surname. I come to this conclusion due to the size differences of East Achronie and Burnside Croft. At East Auchronie, Annie's mother, father and a domestic servant was living with them. There is no mention of William. William would have surely stayed on as a "son" to help with the farm.
Ann Henderson married Robert Bain. I can't find a birth or marriage record that would tell me the names of her parents. Can you help?
They married at Skene in 1878.
Regards
Malky