From the information you have presented, asthma is still the most likely diagnosis. You do not make a diagnosis of asthma or confirm one with an x-ray. It is also possible that your child is still having the effects of a viral infection. This can last for a few months and looks like asthma. If the
wheezing and coughing are persisting there are medications to treat this and the best bet is to discuss this with your physician or see a Pediatric Pulmonologist.
Please read the information on the National Jewish Medical and Research Center’s web site at http://asthma.nationaljewish.org/ for more information about asthma.
We run these all the time on children your son's age in our microbiology laboratory. Are you in the Columbus, OH area? There has recently been an outbreak of Pertussis (whooping cough) there. I'm in NW Ohio, and we've seen an increase in requests for pertussis cultures.
Not all labs can perform these cultures, as they require special media for growth, and they usually need to be collected in special transport systems, which your doctor can request from his laboratory. Most larger labs do testing on nasal washes for RSV. It's a rapid test now - takes about ten minutes once we have the specimen. We do them for kids on the floors and in the ER all the time.
Whooping cough produces paroxysmal coughing and the child can hardly catch his breath. When I was in college I saw a movie in our pathogenic micro class of a child a bit older than your son with this terrible illness. Believe me, he was NOT happy. The poor baby just coughed and coughed and could hardly breath. He was blowing mucus out his nose and mouth. Many times the children vomit up all their food and can't gain any weight, which is when they become dehydrated and end up hospitalized. It's very dangerous for young children, which is why we have vaccines for this. Not sure where your son is in his DPT sequence. If you'd ever see a movie of a child with this, believe me, you wouldn't even think about not having your child vaccinated.
This would be testing I'd ask your pediatrician about, as RSV is much more common than pertussis.
Good luck.