Lived in, worked in, or had relatives in SCA, yeah, that was what I was thinking. The most plausible one to me was that the person he met was a caretaker or housekeeper in the area -- "Hey, I get off work at noon. Meet me there and we'll go grab some lunch and then we'll go get in line so we can be the first ones in."
As far in advance as people buy tickets, it could have been arranged well before the period we have seen phone records for. It could be a date -- the person he's meeting could have invited him to accompany him/her.
After the show, went to an afterparty somewhere. Spent the night with somebody -- maybe the person he went with, maybe somebody he met there. (I'm kinda picturing somebody he used to know but hadn't seen for a long time.) In the morning somebody says, "Hey, I can get you a job with the band." Any of the bands that played, not just Megadeth. Steven looks at his messages. No apology from his father. Nothing but nagging calls from people he doesn't like anyway.
He says, "What the heck," tosses the phone and his old life, and gets in the car.
Every now and then he probably thinks, "I should drop a line to Mom and Dad and let them know I'm okay," but never quite gets around to it.
As for somebody recognizing him -- maybe, but I doubt it. His picture isn't plastered everywhere even in the region and he's a pretty average-looking guy. People just don't pay attention. I bet he could walk through SCA today and nobody there would notice.