Many are saying the videos showed one underseat light dark, and that was his seat. Wasn't the case, there was already an inoperative seat on the ride. The light under the seat doesn't indicate harness engagement. It's just for show. They didn't have them on in the daytime. Apparently there is a light that shows engagement of the harness, although it isn't beneath the seat.
The harness was on and locked, the kid was too big for it to hold him in when the ride started braking and it took his full weight at speed. The seat was tilted at one point, but I'm not sure if it tilts when it is braking, which is when he came out of the seat. There wasn't a failsafe or secondary restraint to help hold him in.
The ride's operation manual puts the maximum weight for passengers at 286 lbs, he weighed 320-340. There weren't any weight limits posted, it was up to the operators to determine if the passenger's weight was over the limit. There wasn't any failsafe restraint on the ride (many rides of this type have a secondary restraint), just the shoulder harness, so it was all on the operators to screen the passengers. Apparently the boy was turned away from two other rides that night due to his size, but was allowed on this one.