It is possible that his service records are stored at the National Archives in Washington D.C.. I found my great-uncle's records there. He died in 1942 after the ship he was on sunk returning from Murmansk. The great thing is that the records will have every port of call he had along with a photo of the Merchant Marine. Contact the National Archives about the records. It may be possible if you do not live near Washington D.C. that they could send the records to the nearest regional Archive so that you may look at the records. Good Luck.