The GAC is not observed on the bus. For that matter it is also not used for character interactions, even those with set queues, restaurants, fireworks (nor is there a wheelchair viewing area), street entertainment, or anything at DTD or the resorts.
Your best bet for waiting for a bus is to go at off peak times of day. Avoid times like rope drop and right afternoon fireworks.
You should know that at a bus stop at the resorts you will DEFINITELY see buses come and not get on them. This is because all destinations share the same bus stop at the resort. You may see several stop, load guests, and leave before you are able to get on your buses
Also good to know is that large parties often have to split up on different buses.
The GAC has a limit of 6 people, so if you are traveling with other parties you will be splitting up often anyway.
If he must have a seat or cannot be touched by strangers, rent a car. The GAC and all the planning in the world cannot help with that one unfortunately.
Agree with all of the above.
The best way to avoid crowded buses is to avoid peak times.
A lot of people leave right after parades, after special shows ( like the castle forecourt shows) or after fireworks, even if the park is not closing yet.
So, if you leave way before those things are starting or wait for a while after they are over, you have a better chance of not coming to a full bus line.
As KPeveler mentioned, you will see buses to different destinations pull up at your resort while you are waiting, so you do need to be prepared for that.
At the parks, the bus stops are fairly close together, so he will see buses pull up and pull away without him.
No you won't need a gac for that they don't use gac for buses at the park stops their are most of the time but not always other bus drivers waiting to switch out another driver you could always ask talk to them thu be more then happy to help you in anyway they can. At night they started puting drivers at park bus stops with ornange vest and lights at cross walks of you like you could ask them any question you like about transportation.
The other problem would be the monorails at mk I am not sure what they can do sinfe it one long que I guess have them to through the exit but the. Your be fighting the crowds coming down and they might not let you wait at the exit. Even the ferrys not sure what they would do those two are no like the buses because the buses each has a que with a place to wait to the side. The ferry and monorails have one long que and if busy it a long line.
I am certain they would NOT let you wait at the exit.
There would be several problems with doing that - for one thing, there are no CMs at the end of the exit ramp, so the CMs at the monorail would have no idea that anyone is coming up, expecting to board at the exit.
The entrances have gates that are closed until the monorail is safely stopped and ready for boarding to keep people from getting near to the tracks. I'm trying to remember if the exit side has gates like that or not - they may not, which would make it very difficult to wait there safely.
Even if there are gates, the doors on the exit side close once the exiting guests are off. There would not be a quiet place to wait where you would not experience the hoards of people getting off (who will not expect anyone waiting at the exit).
Except that it doesn't seem like that would help the situation for the OP. Her son could still be standing there, waiting and watching a bus go by that they aren't getting on. It's not like they could wait out of sight of the bus (since they wouldn't know that their family got to the front of the line if they did that).
Here's what we have done - if we could see as we walked up that the bus would be too full, our whole party waited outside of the bus line for one (or more) buses to fill up and leave. When the line went down enough that we could see our whole party would fit on the next bus, that is when we got into the bus line.
If you have part of your party wait in line, and your child and a companion wait somewhere else, they would have to be ready to come to the front of the line as soon as the rest of the party is boarding. This means close by, stroller folded, ready to get on. I
I agree that if he is close enough to do that, he would also see buses go by without him getting on (possibly even the ones the rest of your party is in line for).
We seldom run into long lines by avoiding peak travel times though.
Disney does not provide a way to "be first" on the bus. Nor is it required to as this would not be "equal access". (
ECV and medical strollers are usually boarded first, but even that is not guaranteed-due to full bus, broken lift etc.)
As many have pointed out, buses come and go past bus stops. At the parks and the value resorts there are formal lines(metal barriers, chains etc). The moderates and up do not have those at the resorts.
I would have to agree, OP your best bet may be to rent a car.
agree
I have heard of preboarding buses where people riding have bought tickets and only as many tickets have been sold as there are seats on the bus (think Greyhound). I have not heard of preboarding buses where people just are waiting in line and might not all get on that bus.
The exception is guests with wheelchairs,
ECVs and special needs strollers. It is faster and safer to preload those guests to allow more manouvering space with less people -and less chance of running over toes.
We have had DD's wheelchair preloaded when the rest of our family waited in a long line when there were so many people we expected to be on different buses. The rest of our family in all of those cases got boarded on the same bus and almost always even had seats.
it would not be fun. 
