Most shipping lines now insist on citizenship, even in the unlic capacity.
Assuming that he decides to go that route, he’ll have to join SIU and as a C card. Currently, the only C card jobs available in the hall are the 1) Pride of America and 2) the Crowley operated MSC fleet (which the A & B cards prefer to avoid), which requires citizenship.
I believe that it takes almost a year now to make the conditional green card permanent and a minimum of another year from the date of application to becoming a citizen, not to mention the time that it takes for him to receive the conditional green card. The OP has a long 6 years ahead of him.