You probably need to talk to your son's doctor right away. It may be different with children, but I would not take more than two nebulizer treatments within 30 minutes in an emergency situation. I'm not sure what you use in his nebulizer, but most of those medications cause your blood pressure to rise and using too much in a short period of time can cause a different set of problems. I am not a medical professional, just a 56 year old female with severe asthma, but I would strongly suggest that you talk with your doctor and find out what the guidelines are on how much is too much to give him before calling 911.
I am so glad he is o.k. It has to be the worst feeling in the world to see your child like that and I'm sure it is hard not to panic. If in doubt, I would never hesitate to call 911, as you saw they were able to start working on him right away. From my own experience, when there is no air moving in the lungs, the nebulizer treatments are just not going to do anything. Of course, you have to try because you don't know that he is to that point. When I'm having an attack, I do better sitting up, I don't know if it would help him any to prop him up on pillows or not. See if he wants any water to drink, if it's an attack in the early stages. It helps me to stay hydrated. In my opinion, your 8 year old is old enough to learn how to use a peak flow meter if he doesn't already have one, which could let him know when he is getting in trouble. If you need me to explain how that works, just let me know. I can not believe your one year old also has asthma.
My prayers are with you and your family,