Which car seat/booster(?) for a slight four year old?

[TheGoop]

I've looked into the law in Spain and think DS will need a booster but not sure which, should we take just the seat style or one with a back?

Can these be taken as hand luggage?

I would use one with a back at that age. No, a high backed booster would need to be checked into the hold. Check with your airline as to whether it would be carried free of charge.

What about a trunki? Rucksack for toys and doubles up as seat.

what weight is he as booster seats/high backed boosters/trunki, etc have a minimum weight limit of 15kgs?

He's only just over two stone so think we might need to keep him in a toddler carseat?