Hello,
You would need to get hold of his form CRS 10 which will hold his service record. It will be held at TNA Kew in BT 382 but I can't tell you which piece unless I know his full name and dob. Those records stopped in 1972 so he should have a file. Unfortunately BT 382 is not available online but he may also have a seaman's pouch held in BT 372. If you don't want to give his name publicly then you can send me a pm if you so wish.
Regards
Hugh