It is a 1-hour window and I haven't heard that you need to be
at the attraction before your boarding group is called, just to arrive within that 1-hour window. And it's only being enforced on TRON; Guardians is still lax about the return time. That said, it has been rumored that
DAS holders are given an extra hour to report to TRON, but I honestly don't know how accurate that is because I've read others who were turned away - maybe they were more than an hour past the time.
Is your son unable to arrive at the attraction until he feels ready? Or just may need to wait to board? Once you check-in at the attraction and get your boarding group converted to a DAS, he probably isn't held to the strict window if he needs another 10-15 minutes before joining the queue or standing back at the boarding point as long as you remain in that area.
My issue with the TRON VQ is that the estimated callback time is way off, with many folks reporting their boarding group gets called as much as 3 hours earlier. I think WDW needs to adjust their estimates to be more accurate.