You signed up in 1998 just to post this? (just kidding! :-) )
Well, that is weird. I'm guessing that you had not seen this kid before, since you didn't say you had. I guess I would just make a point to watch closely for him, and try to observe him without being seen. Try to gather a little info about him - where he actually lives, etc. You said he gave you his name. If he gave you the last name, you might try "anywho.com" to see if a nearby address pops up.
With a kid that age, it really could be anything, running the full gamit from worst case jr. domestic terrorist to merely a hormone crazed, love-struck punk who is trying to spy on the little "hottie" acros the street.
Good luck.
Hey, Mikey Skakel got his start this way.
BTW - if you haven't already, ask your boys if they know him. If he does indeed live that close, he may go to the same school, and/or ride the same bus. They might be able to give you some clues.