This is obviously a concerning and complex problem for you and your family. It is apparent that the Qvar™ Inhaler (beclomethasone dipropionate HFA) is not controlling the level of inflammation for your son. This may be because of the dose or the delivery system. For some 4-year-old children, it is necessary to use a nebulizer with a steroid solution, such as Pulmicort Respules™ (budesonide inhalation suspension), to get sufficient medication in the lungs to control the problem.
As to the cause, this is difficult to say without having much information. The most likely cause of ongoing problems like you are describing is an infection. Mycoplasma is only one of these. It is possible that this is a sinus infection or a viral illness or a non-infectious cause. Further evaluation might be needed to determine the reason for the persistent wheezing.
If your son is continuing to have problems, you might suggest a short visit to National Jewish Medical and Research Center to try to determine the cause and work on better control. At National Jewish, we can offer a multi-disciplined approach to finding out the problem and trying to solve it.