As he's 5, you won't be looking at any of the rollercoasters, so that takes some of the worst queues out of the equation. For the popular non-thrill-rides, such as Buzz Lightyear, you may need to get Fastpasses.
I know that a lot of people find that getting a Guest Assistance Card (GAC) for 'stroller as wheelchair' helps a lot of people with ASD kids. This means that the stroller can be taken into lines, so gives a sheltered 'safe place' away from the rest of the crowd. You can also have the stroller stocked with things to keep him busy, such as a Pal Mickey, a gameboy, a few small toys or some colouring sheets.
A lot of the lines have lots of things to look at, so if it were proposed to him as a walk through the fun line, rather than a queue, this might keep him happier. This would only work on the reasonably fast moving lines, as the slower ones would very much feel like a line. Some kind of spotting or counting game could be good (how many animals can you see? How many circles are painted on these walls? What do all the tombstones in the Haunted Mansion graveyard say? etc), or ISpy if the line is
really slow. Alphabets of things is another good one to do - make a list of something starting with 'A', then 'B' etc. Foods, animals and colours are all good ones to try.
As others said, I think that practicing this in a safe and familiar environment might also be helpful. If you explain to him beforehand exactly why he has to wait (that it's not a punishment; that the other people were there first; that he will get on soon, if he's patient and quiet), and then build up the wait times slowly, he might find it easier to cope when in WDW. Start with something really small, like a five or ten second wait, then work up in gradual increments. You could maybe get some toys to be in the line too, so it makes it more of a game.
Hope this helps!
