Long version: I have a character who is sixteen but looks older, is very strong and doesn't mind hard work, and who has been thrust into the middle of Tokyo without any sort of proof of identity or resources beyond his own wits and strength. I would like him to find, temporarily, a job as a construction worker. Longshoreman would also be workable, but construction worker would be better.
It is essential for plot reasons that he not leave Tokyo. I am willing to fudge the qustion of paperwork by assuming that a sympathetic or short-handed foreman would take him on without asking questions and pay in cash; however, I know in America it can be hard to get a job in some fields without union membership, and my reseach suggests that Japan has, or at least had, a fairly strong union movement which included construction workers. He would be unwilling, for personal reasons, to join a union, even if he could somehow do so without proof of identity.
Would he be able to get a job at all, even "under the table"? If he did, would be be pressured to join a union by his co-workers (who probably wouldn't know about his lack of paperwork)?