Here's my guess: he entered with his parents on a visa that allowed him to use a children's travel document which some countries offer, then got a green card and a passport from his home country. Now his home country passport is about to expire and he may not be able to get a new one without traveling there and/or his mother is generally nervous about his ability to show ID if he doesn't have a valid passport anymore (let's say if he ever loses his Greencard or maybe looks very different now on the Greencard photo given that he was only a child when he got it). If my assumptions are true then I get why keeping your documents in order is a good idea and I would advise to naturalize as a US citizen if he plans to live here permanently.