DS struggles in his new "big boy" car seat

[blossomone]

Hi all,
We’ve recently moved our 4 year old DS out of his Joie 360 spin car seat into a ‘big boy’ booster seat. But he’s being a complete nightmare - the extra freedom of now having an adult seatbelt and not being quite as restricted is meaning he is constantly trying to stand up and reach across to his baby brother etc etc and generally being very naughty! It’s making car journeys very stressful to say the least.

Any tips to get him to stay in his seat nicely?

TIA xxxx

(Just to add. There's no way he could go back in his joie 360. He's a really tall boy for his age)

Is it something like this?

https://www.argos.co.uk/product/2728517?clickPR=plp:7:13

Yes very similar. We now have the Maxi- cosy high back booster seat.

Maybe an extended rear-facing seat that lasts up to 25kg (we have a Britax Maxway Plus which seems good)?

I’d definitely look at an extended rear facing seat. Although legal to forward face in a high back booster using a 3 point belt at 4 years, children often get a poor fit or cannot be relied upon to sit still in the seat and not play with the belt. Children don’t reach a point of bone maturity in their necks to safely forward face until around 6 years.
Have a look at Extended Rear Facing (ERF) Car Seat Safety on Facebook. If you create a post with your sons height and weight, they’ll be able to advise you on a seat which will safely take him to booster seat age.

Not all children are really ready for a booster seat at age 4.

If a different seat would be an option, Joie Bold would fit harnessed forward facing and the harness extends up very high, or there are rear facing options up to approx 125cm, too.

The Joie 360 is one of the taller 18kg seats on the market though so he might still fit into it. I have a 4y8m old, around 110cm tall and he just about fits.

If you won't change the seat, try searching "booster seat training" as this is something they seem to do in America (maybe just on car seat pages!) - doing lots of short journeys on quiet, low speed roads while emphasising the correct behaviour and a small reward after each successful trip leading to a certificate of safety or something.