Do his breathing issues occur because of breathing in hot / humid air, or is it that he has problems when
he becomes too hot? The reason I ask, is because there are various things I can suggest to keep him cool, but which would be no use at all if it's the temperature of the air he's breathing.
The majority of outdoor lines are in MK, particularly Fantasyland and the Jungle Cruise; and in AK. A number of these have Fastpasses, so you could use these to avoid the majority of the line using these. With some of the rides, such as Dumbo and JC, the ride itself is outside, so even if there were no line, he would still be out in the heat and humidity. JC is particularly humid, so watch out for this one.
Animal Kingdom as a whole will be a tough park to deal with, as it doesn't have a huge amount of air conditioning, and a lot of it is outside. Try to hit this on an over-cast day, as that will make it a lot cooler, and will also decrease the crowds.
Do make sure you get the GAC, and if you know a line is going to cause you particular problems, talk to the CM at the entrance. If you're polite and reasonable (as I'm sure you will be), they may be able to find some situation that could work. It may be that they are able to issue 'ad hoc' Fastpasses, or for some lines you may be able to send part of your party through the line, and have you and your DS join them at the boarding point. I don't know whether this will be practical or suitable for all of the attractions, and it will probably also vary depending on the crowd levels.
I also suggest you get him a pediatric wheelchair, or a large stroller, to take some of the strain off him. If it doesn't look 'wheelchair-ish', you will need to get a 'stroller as wheelchair' GAC / tag from GS. You will be able to take this into lines and buildings with you, as well as using the designated wheelchair viewing areas for parades. Try and get him one with a sunshade (or getting an umbrella / parasol to fit on a wheelchair) as this will at least help to keep the air around him a bit cooler. You can also rig up a small battery-powered fan on the frame, so that it keeps fresher air circulating around his face.
Does he use oxygen at all? If so, I would suggest you prepare for the fact that he may need to use more than normal at WDW. You should be able to get a stroller / wheelchair that can carry the oxygen tank with him, and I think you can store spare cans at First Aid.
Other than that, the only thing I can think to suggest is that you take it easy, and keep an eye on him, to see when he's flagging. Space out the rides with outdoor lines, and take air-con breaks in between. If you're doing outdoor shows or parades, you can leave half your party saving the spot, while you take your son inside to keep cool. If he needs to lie down, there are beds in the First Aid Centres that Guests can use if they need to. Just tell them that he has breathing issues and needs to lie down for a short while, and they'll show you to a bed.
Good luck, and have a great trip!