Asthma is really a clinical diagnosis. We see how a child acts with subsequent exposures to viruses and things known to trigger wheezing in kids. Whether "asthma-like" medicines make a difference with the amount of wheezing is also important. The more family members (particularly parents and siblings) with a history of allergies, the more likely the diagnosis. Eliminating exposure to cigarette smoke is always important.
Remember that wheezing does not necessarily mean asthma, and there is no simple, quick test that can be performed to make the definite diagnosis of asthma. Blood tests, chest x-rays, and measuring how much oxygen there is in the bloodstream are usually only helpful with treating the episode of wheezing at hand.
From what you've described, I wouldn't label your son yet. I would treat your son normally with the understanding that he may wheeze and need medication again in the future.