You can lock his belt but you don't have to. It's safe either way. Lots of people lock the belts when their kids are first being "trained" to ride in a booster, to remind them that they have to sit up perfectly the whole time. Just to reiterate, though, and I know this has been discussed to death, if you have to always lock the belt to keep him sitting right, after the initial "training" period, he's too immature and needs to be back in a harnessed seat. My daughter is almost five and is not allowed to ride in a booster, even for short trips in other people's cars.