I can't add much to what you know, it seems -
except a list (attached) of 7th Bn LNLR burials in the area.
It can be seen from it that the Bn (and so, your granddad) indeed was in the Ypres area at several times & places:
they were strongly involved indeed in the Battle of Messines (7 June 1917 onwards), and in the first days of the 3rd Battle of Ypres (31 July 1917 onwards).
The list shows a couple of true battlefield cemeteries (e.g. Croonaert Chapel) as well as the ever-important Hospital Cemetery in Poperinge (i.e. Lijssenthoek).
I hope this list may open up additional opportunities for your personal research...
If ever you're research would end up with names / graves that are found on the Cemetery of Lijssenthoek, we would be very glad to learn more about your findings - so please get in touch then!
Always welcome for further help,
Kind regards from all at TH