Car seat / harnesses

My 5 year old nephew was diagnosed with autism and recently we've really been struggling with him in the car. He sometimes has meltdowns in the car or just generally decides to get out of his car seat - which is so dangerous. 

It's gotten to the point where my sister can't go out in the car because she's knows he'll unstrap himself while she's driving and get in the front seat to hit her.

Because it's half term and she has an 8 year old that wants to go places in the car so public transport isn't ideal.

She's tried different straps that you can't unbuckle, but he's so strong he can get out of anything. I've seen butterfly harnesses but they say not suitable for a car, and he's too big for general car seats now so we're finding it difficult to get the right thing. 

Does anyone know of a UK approved harness, strap or car seat aimed at autistic children from the age of 5 that he definitely won't be able to get out of? 

Thanks in advance