Unfortunately the majority of WW1 Army service records were destroyed in the WW2 Blitz on London.
http://www.1914-1918.net/arnside.htmlI've had a look for your grandad's service record, based on your corrected birth information for him, but I can't see any service record for him.
On the other hand, if he continued to serve after early 1921, his service record will now be with the U.K. Ministry of Defence, but the application fee for that is £30 whether or not his record is or isn't with the MOD.
However, all of the WW1 Campaign Medal records did survive, BUT, unless you have some reason to know that he definitely served in the Royal Engineers, such as for example from personal family knowledge, his surname and his surname and forename or forename initial/initials combination is too common to find a definite match.
He does have a very distinctive second forename, i.e. Thompson, but the problem is that with medal records often only forename initials were listed, and often only one forename and or forename initial was listed, even if the man had more than one forename.
For example,...John William Smith might be listed as...John William ... John W. ... John ... J.W.
You can check medal records on the U.K. National Archives site at Kew in London, it has a much better search engine than ancestry, and the information is transcribed in a much better way and in a more consistent way than it is on ancestry.
http://www.nationalarchives.gov.uk/records/medal-index-cards...IF he served outside of Britain during WW1 he would have been awarded Campaign Medals.
As I said, his name/initial combination are too common if his unit is unknown, however, if he definitely served in the Royal Engineers there aren't so many possible candidates, depending on whether or not both of his forenames or forename initials were used in the medal records.
Presumably his medals aren't in the possession of his family, because his military details would be inscribed on those.
In later life he was an electrical engineer and if he had that qualification in WW1 it would be more likely, although not certain, that he would have been selected to serve in the Royal Engineers.
It wasn't a legal requirement but soldiers who married or had children born whilst in service often listed their military details on the marriage and or birth certificates.
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BTW, I noticed something unrelated in your information,...am I correct in thinking that your paternal grandfather was born in Ireland ?,...because if he was, there is an aspect of that situation which you might not already be aware of.