If you don't want to go to the parks too late, I have found that arriving about 15-30 minutes after park open has much lower crowds entering the parks.
I hope your trip goes well. Our youngest has anxiety, but had always done well at Disney. Our first trip back was in June. Being around so many people after so long not around anyone was very hard for him. It was hard to get him out of the room many days. After that trip we were able to get him medication at the pediatrician for his anxiety. We returned again in August and let him tell us what he was or was not comfortable doing (he is 13). Then took it at his pace. I like that you already have a backup plan to split up and stay at the resort if needed - that could make it a much nicer vacation if he has trouble with the crowds.
I hope your trip goes well. Our youngest has anxiety, but had always done well at Disney. Our first trip back was in June. Being around so many people after so long not around anyone was very hard for him. It was hard to get him out of the room many days. After that trip we were able to get him medication at the pediatrician for his anxiety. We returned again in August and let him tell us what he was or was not comfortable doing (he is 13). Then took it at his pace. I like that you already have a backup plan to split up and stay at the resort if needed - that could make it a much nicer vacation if he has trouble with the crowds.