The best information would be found by getting the birth certificate! But I'll see what we can find...
There is an Alice Maud M Haines b. with the same reference, so he was most likely a twin.
An Eva Myra, bn Tavistock Sep Q 1885 v5b pge 348 is likely a sister.
There is, in 1891, in Tavistock/Calstock/1 Colly Cliff Cottages, a family:
George H. Haines, 34, b. Netstakes, a Stone Mason (employee)
wife Fanny, 32, b. Plymouth
Alice, 7, b. Netstakes
Eva, 6, b. Netstakes
William H, 2, b. Kingswood
(This family appears in 1901 in Calstock with a son William aged 12, so the age on William is definitely right here.)
I'd suggest looking at burials in Calstock (for some reason perhaps his death registration was skipped over but the church burial records would still list him)