the harness is MUCH safer than just the lap belt, google seat belt syndrome.

that said, if you have a low back booster around still, I would use it with the harness until he's 66lbs like the instructions call for. there must be a reason for that weight being listed(you could also call Ezon and ask them the question). Walmart has the harmony literider for $13.50, so you could always pick one of those up to use it.