A bit of a tangled web.
First - his service documents do not show up which indicates they were among the 60% or so of Great War records lost to bombing in WW2. These would not have given detail though but may have at least given dates to work on.
Second - he has a Silver War Badge record (his discharge was 26 April 1918 not 1919) which would fit with a disability such as you describe.
Third - only a representative proportion of Great War medical records were retained and he doesn't show up there. Explained here:
https://discovery.nationalarchives.gov.uk/details/r/C10949Fourth - his medal records possibly need a bit more work.
The Silver War Badge for 512218 has the name Cpl John Ernest Davies of 3rd London FA. The record says he joined on 7 Sep 1914.
There is no medal card for 512218 (the number is a post 1917 renumbering for 3rd London FA).
There is a medal card for 286 Corporal J E Davies discharged on 26 April 1918 (agrees with Silver War Badge) and who went to France on 24 Dec 1914 (Happy Christmas). The 1914/1915 Star roll has JE Davies, the British War and Victory roll has John Ewart Davies and are both for 286 Davies. The 1914/1915 Star roll has the 26 April 1918 discharge date.
I would venture to suggest that your man was 286 Davies, renumbered to 512218 in 1917 with a variety of mis-spellings along the way.
Slight problem is that 85 Field Ambulance didn't go to France until mid January 1915 so the Dec 1914 date looks anomalous - needs more work.
As there will be no medical or service record to refer to, I would repeat my thought that the war diary may hold an answer. Only the first, covering Jan 1915 to Nov 1915 is on Ancestry at:
https://www.ancestry.co.uk/interactive/60779/43112_2272_6-00000?backurl=&ssrc=&backlabel=Return#?imageId=43112_2272_6-00003It would though be the latter diary that would cover his wounding. The division went to Salonika in November 1915 and (we have to assume he went with them) their diary has not been digitised. You need to visit Kew or have it copied)
One ray of sunshine. There is a pension record for him 512218 John Ewart Davies of Wrexham born 1892. This is on Ancestry under the additional subscription area Fold3 to which I don't have access:
https://search.ancestry.co.uk/cgi-bin/sse.dll?indiv=1&dbid=61588&h=1502930075&tid=&pid=&usePUB=true&_phsrc=MvA6435&_phstart=successSourceThat may hold other clues but as it deals with post war events, detail of how and where he was wounded is unlikely to appear.
Hope that helps?
MaxD