DS2 is 6, with ASD. He is also hypermobile and very bendy and squishy. His language is very limited - he's practically non-verbal and functions very much like a large toddler.
Until a year ago, we had him in a Britax Junior traveller. It suited him well in that he couldn't escape from it, but head support was somewhat lacking and we had to use it reclined. He outgrew it, though - too tall so we had to move him into a high backed booster with a crelling harness. This combination was fine for DS1 (also ASD, but with better language and much less squishy). Even with the chest clip, it doesn't contain DS2, though. We had a lovely trip up to the Northumberland coast, today, but had to stop several times each way to put him back in it. He's the same with his walking reins - just wriggles straight out of them unless they're really tight.
So we're stumped. We can afford to shell out over £500 for the Traveller plus, but have no idea if it'll actually fit properly in our car (it's not even on the Britax website so can't use the so called fitfinder) or if it'll be as hard to escape from as the smaller one was. The head support is less of a worry, now, since he's a lot stronger than a year ago and he no longer falls asleep in the car, but it's a lot of money to shell out to find that it's no good (though at a push, I think we could order it from halfords to be collected instore and we could ask them to fit it, but I don't know if they do it for this seat.)
Does anyone have any other ideas, short of duck tape?