Hello and welcome to HealingWell,
From what I have read, children who have a repaired or closed ASD have no restrictions on their activity once their Dr. gives them the okay that all is well post-procedure/healing period. You have a wide time range from what your son's surgeon has recommended and what his cardiologist has said.
Is there another cardiologist at the clinic you could ask re how long before your son should resume his normal activities ?
Wish I had a better answer for you.