Sorry to hear about the hassle you've had with Virgin, but so glad you've managed to get a much better deal. I'm also keeping an eye on prices in case we can book cheaper too, sometimes losing the deposit is worth it for the savings!
My youngest (6) is also autistic. We went when he was 5 and also 6. We found the best advice was to take each day as it comes. Our first trip I tended to plan early mornings for the first week or so and we'd leave the parks after lunch. My little one is a lot younger than yours however we bought a buggy which became his safe space. I also made sure to take his comforters with us (tags, teddies etc) If we felt things were becoming too much for him we would remove him from the situation. EG he found BoG a sensory overload and completely shut down in there so we left early.
I have heard there are rooms around the park that you can use for a sensory break but I haven't read too much into them so can't comment just yet.
The DAS will be invaluable it helps so much. My LO can't queue, it gets him very anxious, but the DAS pass was fab. We would get a return time and have a sit down with a snack or drink until it was our time, or we'd go on a ride that was more or less walk on whilst we waited.
I wrote reports on our trips which are in my signature.
Take a look at the busy days guide on here and keep an eye out for the
park hours release. Try and get to extra morning hours in your first week. These are only available for guests who stay at Disney meaning it's much quieter and you can get quite a few attractions done.
In regards to clothes we pretty much lived in shorts and vest tops/t-shirts. We also had comfy trainers (sketchers are fab) I'm also getting some sketchers sandals for next year.
QSDP - there are a few restaurants you need to book with QSDP.
Be Our Guest - You can book for breakfast and lunch on the QSDP. We generally book for breakfast on our first morning and a lunch later in the holiday.
Pizzafari - Again this is another you can book for lunch or dinner but you can also just walk in and collect your food.
Restaurantosaurus - This is a new one but it's burger and shake I think. We'll be booking this to try this time.
There is another but off the top of my head I can't remember what it is!
The QSDP is fab, it's not just burgers and chicken nuggets, so many choices. Take a look on the Disney website and you can find all the menus on there.
If you have any questions feel free to PM me
Xx