Same situation here, my daughter was not diagnosed at a young age due to slipping through the cracks. Once she hit puberty she really changed, seemingly overnight. My son has had a diagnoses since he was 5 or 6 so he has been able to get much better support at school. However he exhibits much more "traditional" ASD traits than my daughter. We never got the
DAS pass for my daughter as she didn't need it but our son for sure does.
All you have to do is go to guest services at any park on your first day. Then tell them you require a DAS pass for your son and give a brief description of why he will be unable to stand in line. They will then scan his band. The DAS pass is good for all parks throughout your stay so you don't need to visit guest services again. Once you are inside the park you will need to walk up to the CM at the entrance to each attraction and tell them that you have a DAS pass. They will then scan your sons band and then you can come back with your party at the designated time. Each person will have to scan their bands when entering.
Usually wait times are similar to what is posted, but sometimes they can be much shorter. Regardless you are only allowed to have one DAS reservation active at a time.