I am a former Marine and I served in Hansen in Okinawa for 2 long years. Here is what you do:
Get a digital camera with a timer and take A BUNCH of naked pics of yourself in all sorts of naughty poses and angles and send it to him.
He will be so thankful and appreciative not to mention he won't be able to stop thinking about you.
Best gift I EVER got.
There is absolutely nothing he cannot get in Japan, either on or off the base, that he could get in the United States. This is true, with some notable exceptions, primarily, you.
As for the perfect birthday gift... Be creative. Send him a handwritten I.O.U. for one of the most interesting nights of his life. Payable upon his return to the U.S.
"One of the most interesting nights of his life" can be whatever you choose to make of it. You have two years to think about it. Your I.O.U. will keep his interest for the two years he is away. He will plaster it to the inside of his locker, along with your picture, and see it everyday. He will proudly insist that every Marine in the barracks see his I.O.U. You will make him the envy of his platoon. Of course, the I.O.U. is far more hype than reality, but you cannot imagine what it will do for your Marine.
Along with the I.O.U. you can promise to send him a handwritten note every day. It might only be one or two sentences, it could be several pages. Getting real mail rather than EMail will do wonders for your Marine. You might get a very pleasant surprise, handwritten return mail. As a matter of interest, the postage cost is exactly the same as you pay for regular domestic mail.
You have an opportunity to make his tour of duty a memorable and enjoyable assignment while enhancing your relationship.
P.S. You might want to reconsider the suggestion to send photos of yourself on a digital camera. Since 9/11 things have changed, it is easier to be detected. It is a crime to transmit such material via the U.S. Mail. A Marine in possession of such material would be subject to a courts martial. It is not possible to keep such items under lock and key
send him some American clothes! there are probably none over there! hmm.. or some american candy japan might not have? something american he misses. or a poster of two naked chicks. yeah!