Yes, but the ADA's purpose is designed to level the playing field between those with disabilities and the general population. For instance, I am on an insulin pump and would have to avoid rides whose queues were in direct sunlight in order to minimize my risks of my insulin spoiling. Under the ADA, a reasonable accommodation that could be granted by the
DAS is the ability to wait the same amount of time outside of the queue; getting front of the line access would not be a reasonable accommodation, as it creates an uneven playing field that favors me more than the general population.
Quite frankly, asking for an accommodation where the height of the child is predetermined is a major safety and liability issue for Disney. We tend to shrink slightly during the day, and, in the event that a tragic ride accident occurs because of the child's height, Disney sets itself up for lawsuits and a boatload of OSHAA violations. Personally, I do not see how having a pre-measured height can level out the playing field for someone with a disability. It is not like the current height-measuring standards (the L-bar) creates an uneven playing field between the general population and the OP's son, as the son is able to be measured using it. The requested accommodations are simply for convenience (it does not affect the child's ability to ride a ride compared to other children at a similar height), and, as previously stated, could open up multiple lawsuits for Disney if something were to occur, and it is identified that the child was too short to ride (even if it was by a half inch).
Training staff to identify that a height written on the DAS (or a band placed on the child's wrist) is legitimate could be considered a financial burden. It is even a greater financial burden once people begin to catch onto this new accommodation, begin to abuse it (such as writing a falsified height on the DAS a few hours after receiving it), and open up more lawsuits for Disney.
To the OP, I highly recommend measuring him yourself right before you leave for Disney. If he is on the border between being too short and being tall enough, it might be a good idea to avoid those rides and prevent being in the situation where you tell your son that you are going to ride a ride, you later learn that he is too short, and he has a meltdown because of it.