If it was a regular season game, I would say get there EARLY and make your way down by the tunnel heading to the locker room when players are on their own shooting around. I'm talking an hour before tipoff. They sometimes go to the crowd and sign autographs. During the season, he would have a pretty good chance to get a couple players.
This being the Finals, though, he would have a much lower chance. Given the importance of the game, most players are completely focused on their warmup routines and game preparation and signing is probably the last thing on their mind. Add in the extra security, media presence, and packed house due to the hype of the Finals, and it will be tough to get near a player once they're in the building.
So tell him not to be disappointed if he can't get one tomorrow. He might not get an autograph, but how many of his friends at school can say they saw a Finals game in person?