Well, you are supposed to supply proof of residency on that one if asked. It's possible that they won't even ask, but I think you're wise to be prepared.
We just got DS's report card and it has our full address on it. Maybe you could send him with a copy of his report card, or any official communication from his school that has your zip code on it, and his student ID. (I assume his student ID has his picture on it?) I would think that would be adequate for a teen who is not traveling with a parent, doesn't have a DL yet, and is hopefully not receiving stacks of official bills in his name.
Honestly I think they'd probably be OK accepting a chaperones' ID too, but at 15 would your DS prefer to handle it on his own rather than needing an adult to vouch for him?
The last time we used multi day passes with a place for a signature it wasn't until our last day that we ran across a CM who actually asked us to sign them. I don't know if they're any stricter about that now or not, but either way that shouldn't be a big deal, he can just sign his own.
Good Luck getting him back to the parks with you. At that age sometimes you just have to twist their arm a bit to get them there, but once they are, they usually have a great time.
