I think this is one of those situations where, as a parent, you need to work with your child on understanding that this ONE ride has a special rule. I have 2 ASD kids. If there is one thing they understand, it's rules. We also deal with extreme anxiety, so I totally understand how that goes.
We had a similar situation a couple weeks ago at Universal Hollywood with Super Nintendo Land and the Mario Kart Ride. We did get a Guest Assistance Pass. However, the rule with that land is, once you get a return time for the Kart ride, you CANNOT leave the land and come back in UNLESS you have another land entry reservation for later, and they go QUICKLY. The return time DOES NOT grant you entry to the land if you have left it. The line is often 3 hours long, so that means getting a return time and sticking it out in that tiny land for 3 hours, potentially. We talked about it with our sons at length beforehand, and explained that *IF* we got a return time, we would have to ride it as soon as we were able, because we may not get back into the land again later. They both understood, and were ready to board as soon as our time came up.
With Tron, since you will have a DAS, once your boarding group is announced, you can return to convert your VQ to a Return Time, and tap in.
Walk into the line a bit, until you are sure you are past all the tap points, and move off to the side if your child needs a few extra min. Let others pass ahead of you. You can probably hang out in the
Lightning Lane for 10-15 minutes for your child to "get ready" if you need to. I'm sure if a CM tries to move you along and you explain the situation, they will leave you alone.
I feel very strongly that, as parents, it is our job to teach our kids how to navigate the world and its rules, and while we do use a DAS, we are always also teaching the lesson that very often, there are times where special rules apply, and we have to be able to function within those special rules, just like everyone else does. The DAS is already giving you the option to use the LL rather than wait in the VQ line, so that is *already* the accommodation. The requirement to return within an hour IS reasonable. If you obtain a VQ, just be in the park earlier than you expect to be called, so your son can take that hour to get mentally prepared.
The other option, of course, and the one I plan to use when we go this summer is to purchase the ILL and choose a time that generally works well for your son. This way, your son has a specific timeline to work with, and can start mentally preparing ahead of it.