Hi welcome to healthcare magic,
ASD i.e Atrial septal Defect
is usually asymptomatic if small .
If moderate will present with symptoms later in life usually by 40-50 years age.
Is severe he would have symptoms by now.
So size of ASD matters a lot and not everybody follow this rule, exceptions exist.
The best thing would be take a pedicatric cardiologist opinion , see if he advices surgery ( the surgery is not a open heart surgery , its a simple one which they do it by passing a wire through a vein , with good success rate ) and if not just follow up with him yearly or every 6 months to see if the size is increasing.
No special precations are needed , except for get him adequately treated if he gets any throat infections....
Hope this was informative .