Car travel problems

Hi,

I wasn't sure where to put this question, so I'm sorry if it's in the wrong place!

We're going through the diagnosis process with our son, who's 4. We've been referred to the pediatric consultant by both the health visitor and GP. Anyway, in the mean time, we're having huge trouble travelling in the car with him. He has no sense of danger and repeatedly unclips the harness on his car seat, climbs out of it, and a couple of times has tried to climb out of the window. 

I really need to get him a specially adapted car seat, or some sort of additional restraint to stop him from causing such a danger to himself!

Does anyone know of any good products or car seats? And are we able to claim DLA to cover to cost of any of these products? I've found a couple of car seats online, but they're £400+ which is too much for us to afford without help.

I appreciate any advice you can give!

Becky Relaxed