You can request his military service records from the Personnel Records Center in St. Louis. The website explaining how to request the records is https://www.archives.gov/veterans/military-service-records
Dear Ms. Twomey,
Thank you for posting your question on History Hub!
We suggest that you request a copy of his Merchant Marine Personnel and Licensing Records in the Records of the U.S. Coast Guard (Record Group 26). The Merchant Marine Personnel and Licensing Records dated prior to 1968 are in the custody of the National Archives at St. Louis (RL-SL). Please contact RL-SL via email at firstname.lastname@example.org. Include the name, date of birth, dates of service, "Z" number, and date of death, if known, in your request.
Due to the COVID-19 pandemic and pursuant to guidance received from the Office of Management and Budget (OMB), NARA has adjusted its normal operations to balance the need of completing its mission-critical work while also adhering to the recommended social distancing for the safety of NARA staff. As a result of this re-prioritization of activities, you may experience a delay in receiving an initial acknowledgement as well as a substantive response to your reference request from RL-SL. We apologize for this inconvenience and appreciate your understanding and patience.
We hope this information is helpful. Best of luck with your family research!