As the birth is after 1911, cant use any census to help establish his parentage, so his certificate of birth will prob be need - bonus is that after 1911/12, his mothers maiden name will be shown in the index, so might be able to deduce a marriage of his parents from that.
Anyways ... I'll see if I can find his birth index entry. 
so now to see if theres an obvious marriage
any thoughts on these 3 Male Cox to female Coventry possibilities - ie both names appear on the Index page?
Marriages Jun Quarter of 1891
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Cox, William Pancras 1b 89 possibly to Susan Emma Coventry
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Marriages Sep Q of 1898
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Cox, Edward
James St. Geo H. Sq. 1a 1069 possibly to Charlotte Katherine Coventry
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Marriages Sep Q of 1911
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Cox, William Fulham 1a 575 Possibly to Constance M A Coventry