Help- seatbelt issues

[Yamayo]

My 9 year old severe autism and we find long car journeys tricky.
We have a safety harness which is essentially a 5 point clip thing for older kids but it has to be used in addition to the seatbelt to be safe.

Issue is that he occasionally gets very stressed and pulls the seatbelt as far as it goes (he hasn't worked out how to unclip it yet) and slides it under his bum, which means he is not safely secured in the car anymore.

Can anyone recommend some kind of clip I can add at the top of the seatbelt to stop it from being pulled loose?

Any suggestions most welcome!

Sorry to bump but was wondering if anyone had seen this?
We've just had a massively difficult car journey and I am getting desperate!

We have this problem too. I've tried loads of different things that haven't worked but decided to have a google and found this:
https://www.amazon.co.uk/dp/B07DVLWN6R/ref=cmswwrcppapi_zdTFBbMK5Y9HZ

Think I might get one too!

Thanks for the tip! Will order it and tell you how it goes.

Have you looked at a Houdini harness? I think the manufacturer is crelling, these are very good for escape artists!

Creilling harness is most likely the safest option.

We have one of those but the manufacturers instructions mention it has to be used with a seatbelt as it's not safe by itself.
He was fine with it and seatbelt for quite a few years but he's only recently started pulling the seatbelt under his legs.

I wonder if any of these products would work as a car seat for older children - taking the belt out of his reach and the equation? I expect they are expensive, but might be worth a look. If you don't know his weight offhand, my 10yo is 32kg as a rough guide.

incarsafetycentre.co.uk/category/special-needs/

Have you tried putting the seat belt underneath the houdini strap part of the harness? It makes it more difficult to pull it.