You've been given some great advice, J.. If those approaches don't work, and you can find the patience for this (considering you're seldom if ever getting medicine into your son anyway), you might want to try reducing your forcefulness and putting MORE power into your son's hands. I find this is usually effective and stress-banishing with my grandboy, who has just turned 2. But you may have to give it some time.
Ask your son, while he's having difficult symptoms, if he likes what's happening, or if he likes how it limits his ability to do things he likes. IF he says no (and this won't work if he doesn't care that his breathing is rough), ask him to suggest some things that might make him feel better. You can coax some yes/no answers by brainstorming with him. Make some ideas silly and fun, like, "Do you think you'd feel better if a flock of parakeets landed on your head? …if you sit in a warm bath? …if I tickle your sister? …if Daddy eats your shoe? …if a fireman brings you a fire truck full of balloons?" …etc.
If he gets on board, and he might after a number of tries if you keep it low-pressure, ask him if he's noticed he feels better when he takes that ICKY!!! medicine (be cheerfully sympathetic) and then tell him you'll be happy to let him take the medicine (use the nebulizer, whatever) if he wants to feel better, and that he should think it over and let you know. Then change the subject.
It may seem counterintuitive, yet empowering children, especially a strong-willed child, can make a world of difference. And you'll have to be willing to let it take whatever time it takes. You say your son "really needs" this medicine. Might it be more accurate to say he would really be helped by this medicine, but he is apparently surviving without it, so YOU are the one who needs him to take it? His symptom might be tormenting you, but merely annoying him. Could that be as true? It is so hard to listen to what we interpret as suffering in our precious little ones.
I grew up with asthma myself, and learned by my early 20's that feeling pressured, forced, and disempowered simply made my wheezing worse. (I'm amazed now that it took me so long to figure this out.) There was always a physical trigger, but anxiety, frustration or anger could turn a tickle into gasping desperation, and of course my poor mother would get desperate for my breathing to ease, and redouble her efforts and her intrusiveness. This modern medicine wasn't available when I was a kid (I'm 60 now), but I can't help but wonder if that wouldn't have seemed like one more layer of torture I had to endure at her hands.
If you can get your own need under control, it might make your interactions with your son much less stressful, and give him a chance to cooperate. It sounds as though he probably doesn't have the space to to that now.