What are the rules on changing from a car seat to a booster seat? My son is 3 years 4 months and weighs 2stone 6lb
https://www.gov.uk/child-car-seats-the-rules/using-a-child-car-seat-or-booster-seat At 15kg your son is (just) legally able to move to a high backed booster. But I wouldn't do so unless he has outgrown his stage 1 seat as it will be safer.
I would check the weight limit on your current seat, I think ours goes up to 18kgs. My eldest is still in his stage 1 seat, he's tiny (for both height and weight) so I think we still have a bit of time left in the seat. They recommend high back boosters for as long as possible x
