The war diary for 6 Battalion Borders for the period is at the National Archives in its undigitised form
http://discovery.nationalarchives.gov.uk/details/r/C7357691. It is also on Ancestry if you have a sub.
The battalion had landed in the Helles sector on 21/22 July 1915. After some time in that sector, the battalion took ship and were moved to a point south of Suvla Bay where they landed and had a week's rest. On 7 August they move into the firing line for an attack to take place on the 9th. There was heavy fighting all day that day until about 5.30 in the afternoon when the battalion were forced to retire having lost 12 officers killed, 4 wounded and 1 missing (of a total of 20) and 26 ORs killed, 241 wounded and 131 missing. The battalion had started the day with 696 men.
"Soldiers died n the Great War", the Register of Soldiers' Effects and the CWGC all have his death as 9 August.
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Added - He is commemorated on the Helles Memorial which implies that he was among those posted missing and that his burial place was never found.