My 2YO has asthma. It started when he was about 6 months old. He HATED the nebulizer and only just started to do it willingly when we have the proper distraction (he loves watching Curious George).
My little guy is VERY active and usually very healthy, but it flares up in the winter. He's triggered by the cold and viruses. He cannot spend a lot of time outside in the cold air (no sledding for him), and we have to watch out when he gets a cold.
His symptoms do not slow him down or seem to bother him, but I can tell when he's struggling to get a good breath.
He is on daily Singulair, has Pulmicort for when he gets a cold to prevent asthma symptoms, and Xopinex to treat the symptoms when they flare up. He's already had pneumonia once this winter and had to be put on steroids (in addition to his nebulizer treatments and antibiotics) to get his asthma under control. He had one asthma attack last year that required steroids also.
I won't speculate as to whether your son has asthma or not, but if the neb helps, why not use it?
I am grateful that my son has the medication and medical care to help him breathe easily and keep him alive in this harsh weather. His dr. said he will most likely outgrow it, as did my husband, brother, and sister.