If he was a Royal Navy Commando then you would have to apply to the RN for his service record:
RN Disclosure Cell, Room 48, West Battery, Whale Island, Portsmouth, Hampshire, PO2 8DX
There are 2 forms to be completed, the 1st is RN/RM search document (part 2), as well as Part 1 (next of kin form). An explanation about them and the downloadable forms are here:
http://www.mod.uk/DefenceInternet/Ab...yalMarines.htm
There is as has been said a fee of ?30. There can be a lengthy delay in receiving them and the information varies considerably from record to record. Some very good others very disappointing. It's worth doing.
Regards
Pete Rogers