I presume the record you have is his Seaman's Pouch held at Kew in piece BT 372/880/118 -
https://discovery.nationalarchives.g...ils/r/C8272928

The fact that he has a Dis.A number means he did serve in some capacity in the Merchant Navy. The pouch is hit or miss but you really need his service record which begins Jan, 1941 and is held at Kew in piece
BT 382/1891 - https://discovery.nationalarchives.g...ls/r/C10990116
This file should give the names and dates of all the ships he served aboard including ports of engagement and discharge. The file is unfortunately not available online and will need a researcher or visit to Kew to obtain it.
Regards
Hugh