The disABILITIES FAQs thread (near the top of this board) has one section about air travel. Some of the links in the air travel section are about the Orlando airport, from the Orlando airport. There are pictures of the airport that might help you.
Here are some pictures I took of the DME 'experience'.
Buses are routed depending on which resorts guests are going to. One trip, you may have guests from 3 resorts on the same bus. Your stop may be first, middle or last. I don't know if this would work, but when you check in, you could ask if they would be able to stop at your resort first.
This is the area where guests wait for the bus. Guests are assigned to different waiting rows depending on which resort you are going to.
Because some people need a special bus with a wheelchair lift, there is a waiting area at the front of this waiting area where people with wheelchairs/
ECVs wait to board. It has some benches and you are not waiting with other guests. That's where I was sitting to take the last picture. If you think it would be helpful, you could explain your son has problems waiting in lines with others and ask if you could wait in the wheelchair waiting area instead.
The bus is very high off the ground because it has space for guests to store their carryon bags in compartments underneath. Since it's so high, there is a steep set of stairs to get on and off.
This is the inside of the bus and the seats. I was sitting in the back of the bus, looking toward the front.
There are TV screens hanging from the ceiling of the bus. There is some storage for belongings above the seats.
The movie stars Mickey and many of his friends!