Are you sure that you can't unthread the straps, remove the extra insert/padding and re-thread the straps at a higher level ? Most baby seats have more than one slot for the should straps.

And similar advice to others, they can use the seat until their eyes are level with the top of the carseat.

Most toddlers don't put on much weight from 2-4 years, so a 16kg 2 year old may well be an 18kg 4 year old.

My godson was in a very nasty car accident when he was 4. The rest of the passengers sustained very serious injuries (it was a tragedy) but he was unscratched because he was in a proper child car seat with a 5 point harness in the centre position of the back seat of the car.

There is no way that a 2 year old is safe in a booster seat. If you have friends that will loan you a better baby seat, then go for it... it should be a 'no-brainer' for your husband, surely he wants his son to be as safe as possible ?