Thanks for the query
The course of pulmonary stenosis
depends upon severity. In mild pulmonary stenosis does not progress usually. Moderate and severe pulmonary stenosis tend to progress with age. As your son is diagnosed as a case of moderate one, this is the reason your doc has advised for repeated evaluation. The decision of surgery can be made only when child becomes symptomatic or when criteria needed for surgery suggested by ECHO or cardiac catheterisation. Surgery usually done is balloon valvuloplasty( dilatation of stenosis with balloon without open heart surgery).
So follow the advise of your doc and go for serial evaluation.
Wish you and your family good health