HMac, is correct. The document you need is his form CRS10
It should be here.
http://discovery.nationalarchives.gov.uk/details/r/C10988554If he had MN service after WW2 his service may be recorded here.
http://discovery.nationalarchives.gov.uk/details/r/C10990418Together with BT 391/18/88, pre order both documents to be on the safe side. Give as much
relevant info as you can. DOB, Discharge A number. Leave it until afternoon before you visit and with any luck TNA staff will have already copied the correct documents.
CRS 10's are stored alphabetically in blocks of 60 names there for you cannot access an individual seaman. CRS 10's contain sensitive personal information and I suspect, been used in the past to commit Identity Fraud. TNA take security to the higher limit when they receive requests for CRS10's. Your best bet is to visit Kew, trying to get the National Archives to search for you can be an expensive nightmare.
The procedure is somewhat complicated but TNA staff are very helpful and you should not have any problem. After you have obtained his CRS10 you can then go on to view Crew Agreements and Logbooks it all depends how deep you wish to go with your research,
It should contain the following information
The front page of the service entries includes the following information:
•Name •Date and place of birth
•Discharge 'A' number
•Rank or rating, with relevant certificate details
•Details of any other qualifications
•Date CRS 10 first page was created
•British Seamen's Identity Card Notations. The entries refer to correspondence and issuing of cards, certificates, applications etc. The originals in general do not survive except where they have been filed in the seaman's pouch. See below.
•Merchant Navy Establishment [licenced] Agencies (MNEA) Contracts
•The continuation pages show: A list of ships and their official numbers with date and place of engagement,
•rank or rating,
•F or H (for Foreign or Home trade voyage),
•date and place of discharge from the ship
•character grade
•National Insurance contribution code
•Registry clerk's initials and date of the entry.
•When ashore the sheet may bear the entry MNRP (Merchant Navy Reserve Pool) with date and place.