First of all, my son only took naps until shortly after his 2nd birthday. He just didn't need them anymore. I figured this out because he went to bed at 8, and normally fell right asleep. For a couple of weeks though, I was laying him down and he was still awake as late as 10 some nights! I decided to cut out his nap and it was just what he needed. He began sleeping all night from 8 until 8-8:30 the next morning. So, the kindergarten thing you mentioned isn't what's right for all kids. My daughter took a nap until she was about 3 1/2, and I quit laying her down for one for the same reason.
As far as what to do if you really think he still needs his nap, you're going to have to not give into him. So what if he's saying he wants down? If he said he wanted to eat a whole gallon of ice cream would you let him do that? It's not up to the child what happens, and I think that many mothers today think that if a child doesn't want to do something, it means they can't be made to. My son went through a period where he didn't want to sleep during his naps, he just wanted to play. So, I laid him down, closed his door, and laid on the floor outside his bedroom so I could peed in the crack under his door. Everytime I saw a foot come down from his bed I would open his door, lay him back down, and leave without looking at him or speaking to him. He'd cry for awhile, call for me, and eventually try to get back out, but I would see his foot come down and repeat going in and laying him back down. I kept doing it until he fell asleep, the first day it took about an hour and a half. The second day I only had to go in 4 or 5 times, and the third day only once. You have to teach your child his limitations and your expectations. Without doing so, they are not going to do what is expected of them, and instead only do what gets them what they want. Be consistent and you will never run into issues with your child that can't be controlled.
Oh, and the reason I didn't talk about the breathing treatments is because I don't think the two are related. My son is asthmatic and takes not only albuterol, but sometimes prednizone (a steroid) and even more, sometimes he takes an oral liquid steroid. The breathing treatments I always do right before bedtime so they help throughout his sleep and they've never made any kind of difference in his sleeping pattern. Also, my sister gives her two children breathing treatments everyday, even when they're not showing symptoms, as a means of recommended treatment from their pediatrician, and she gives the treatments before bedtime also.