When you view the diary you will see that the Dorsets were at Hill 60 on the 5th.
At 8.35 am. that day the Germans laid gas nozzles in front of the British trenches.
Your man would have been one of those casualties.
This was one of the first occasions that gas was used during the war.
The 1st. being in April at St. Juliaan on the Ypres salient.
It was also the place where the largest man made explosion took place the
following year when 25 tunnels were dug into the hill & blown up.
Below is a map of where he was.
The British lines are blue & Germans red.
The side-by-side map shows you then & now.
Hill 60 is in square 29 between the railway track & Zwarteleen.
https://maps.nls.uk/geo/explore/side-by-side/#zoom=16&lat=50.82435&lon=2.92728&layers=101464897&right=BingHyb