Generally, wait-staff will be assigned to a new ship with every contract. Most crew-members move around a lot. Will shoreside be able to tell you if he is on board? I doubt if it is easy to find out such information - they might but certainly no guarantee. It depends on what cruise line it is, and what position it is. Can you imagine how many waiters Carnival has?
Another factor is that you would need to know his exact first & last name. Shoreside people have never met 99% of the ship staff, so they don't "know" them.
The best rule of thumb is that if you want to stay in touch with a crewmember is to ask them personally the best way to do it. And e-mail is usually the best bet if the crewmember has it (many of them do now).
You could try writing a letter to the crewmembers name in this form:
Firstname Lastname CrewPosition
c/o Cruise Line Name
Cruise Line official U.S. address
and I would guess they might find him and get the letter to him - no guarantee, though.