Hi
The marriage cert would be an investment, as it should name his parents and put you on the right track among the COPLANDS.....however, a look at the Census does provide some clues:
By 1901, not sure if William has died? or is absent from the house on Census night? His mother Fanny, appears to be in the household though:
1891: 15 Jamieson Street, Glasgow Govan, Lanarkshire
Head: William COPLAND 36, Loco Engine Painter, b Castle Douglas, Kirkudbrightshire
Wife: Catherine COPLAND 34, b Sanquhar, Dumfries-shire
Son: William COPLAND 13, Stationers Message Boy, b Glasgow
Son: James COPLAND 2, b Govanhill
Dau: Margaret COPLAND 1, b Govanhill
1901: 119 Smith Street, Glasgow Govan, Lanarkshire
Head: Catherine COPLAND 43, Keeps Boarder, b Sanquhar, Dumfries-shire
Son: James COPLAND 12, Scholar, b Glasgow
Dau: Margaret COPLAND 11, Scholar, b Glasgow
Son: Thomas COPLAND 8, Scholar, b Glasgow
Dau: Fanny COPLAND 5, , b Glasgow
Mother: Margaret DARG**EL 71, b Sanquhar, Dumfries-shire
Mother: Fanny COPLAND 71, b Colven, Dumfrieshire (Colvend?)
Plus 3 young male boarders: McLEAN, TIUTT, FORREST
William's age in 1891 indicates a birth of around 1855. Also noting the son William in 1891 - born a decade or so before he married Catherine.
It also seems he was married before he married Catherine - 1881 Census coming up...
Cheers
AMBLY