A full name, even if it's a guess, is a lot more to go on than a shortened nickname. The world is a big place, and there are like 130 million Japanese people in it.
One approach would be to try to go back through your own correspondence and try to find contact information that may still be valid. e-mail addresses, mailing addresses, phone numbers, etc.
Other identifying information would be things like where in Japan he was from, how old he would be, what kind of work he did/what he was doing in Manila. Information about his family, or interests, or associations through his hobbies could also be leads. If he ever competed in a competition or some other event that would have a roster of participants... You're the one that knew him and talked to him and exchanged correspondence, it's up to you to provide identifying information if you are serious about finding this person.
Otherwise, just cherish the memories, and use it as a reminder not to get too busy to curate your personal connections to other people.