Oh yeah, the boy has been in the show before, but I don't think he was with Elliot at all during that day, I think his mind created all the interactions with him after he saw him earlier in the day. His parents were preparing to leave town, why would they leave all day to a place that's a 2 hour drive from their home? That doesn't make sense to me. Also, it's too much of a coincidence that the boy is so adamant about going to the cinema the exact same day Elliot wanted to go watch Back to the Future. You also don't see the boy interacting with anyone other than Elliot since they meet in the beach.