Martin doesn't say his enquiry is historic. The information won't be public if the person of interest is alive. It is possible to ask the Ministry of Defence, but personal information isn't handed out with the rations. Solid reasons and bona fides required.
There is a free database of WW2 Royal Navy deaths.
Otherwise, for historic people, Ancestry.com and similar sites are the best bet. When I used Ancestry about 10 years ago they had very little apart from census data but since then lots of other data has been added, including military records.
Unfortunately probably necessary to take out a subscription and certainly necessary to learn how to use the site. The sites are good fun when tracing a family tree as an interest, less suited to finding one-off individuals. Rarely as easy as typing in a name and pressing Send. More like puzzle solving, where clues are followed until the individual is identified. Unusual names are much easier than common ones, and it's surprising how often people in the past wandered away from their birth certificates.
Might get lucky and find a forum member actively researching their family who could do a search for you? Otherwise, hope a free trial will find the chap, or cough up for a subscription
Dave