No, birth certificates weren't required in order to enlist,...a recruit merely had to claim to be, and appear to be, 17 to join a Territorial unit, and 18 to join the full Army, but they weren't supposed to serve overseas until they were 19, but of course many men did enlist and serve overseas whilst underage.
Why did he at some point give his birth surname ?, well until adoption was introduced in the late 1920's there was no legal provision for the parent of a second marriage to legally adopt a child of the first marriage, and to legally change the child's surname via adoption, so his birth surname was still his official legal surname, although in Britain a person can use whatever name they wish to, provided that they don't use it for criminal purposes, and provided that they use their legal surname for official purposes such as marriage, although even in that case they can marry under their unofficial surname, provided that they also declare their official surname, in which case they will be listed on the marriage registration as, Smith commonly known as Brown.
However, he was warned in writing on his enlistment papers that he was committing a punishable offence if he gave any false answers on his enlistment form, and perhaps he was concerned that he might be found out, and he wanted to correct his situation.
Perhaps he discussed his true parentage with someone and his superiors heard about it.
Soldiers were encouraged to make a will and perhaps he was concerned that his will might be invalidated if his real name was subsequently discovered.
The automatic beneficiary would have been his step father and perhaps he wanted to make his mother the beneficiary and wanted to ensure that his step father couldn't challenge the will.
Perhaps he had applied for a war disability pension and he had to do that in his birth name because those pensions were administered by the Ministry of pensions and might have been linked to his National Insurance documents which might have been issued in his birth surname, some men did serve under a false name but claimed war pensions in their real name.
When a man claimed a war pension his service records were sent to the Ministry of Pensions and perhaps that was the record that you saw, and if so perhaps his surname was chamged after his service, if that was the case and it happened several years after the war, his medal records would probably still be in his step father's surname.
http://www.nationalarchives.gov.uk/records/medal-index-cards-ww1.htmSometimes when a child is raised by a stepfather and their relationship is a good one, the child will continue to use their step surname as an adult, and will even sometimes legally change their birth surname, but if the relationship is a poor one, they may revert to their birth surname as an adult.
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