If your son sits facing the windscreen in the front with a child booster seat there is no need to turn it off matey as it's ok due to him being far enough away from the dash and the effect of the airbag going off will not be as powerfull as it would be if he was in a rear facing seat due to him being too close to the airbag itself, My son sits in the front of mine and i just make sure the seat is as far back as it'll go and the seat belts adjusted to his height on the B piller so fits well.
I asked about this at a few places before i allowed him to sit in the front and that was the advise i was given even though most did say the safest place for children was in the rear of the car but they love to sit in the front

Sorry i dont know where the switch is as i have an is200 but somebody will hopefully know and be able to help on that one?