There are a couple of Morgan Lewis' born around 1877. One appears to have William as his father and another on the 1881 Census with parents David and Margaret Lewis.
1881 Census 3, Hedge Hill Row, Trevethin with Pontypool
David Lewis born 1834 Aberdare blast furnace worker, iron
Margaret, wife, born 1844 Aberdare
William, son, born 1873 Trevethin
Mary, daughter, born 1875 Trevethin
Morgan, son, born 1877 Trevethin
Watkin, son, born 1880 Trevethin
RG11/5252/79/16