What seat is he outgrowing? At 5.5yo I'd be looking at a harnessed seat rather than a seat belt booster.
I’d definitely consider that also. He’s tall for his age- the size of an average 7yo, hence why I posted in this thread. He was 120cm when he turned 5 (the height of OP’s child at the time she wrote this) and nearly 7 mo later now, I know he’s taller still now, though not sure of his current height.
Is there a harnessed seat you’d recommend for him?