Perhaps ask the cardiologist first for a list of medications your son cannot take because of his heart condition. Give this like to the pdoc so the pdoc can make an informed decision.
I think it might be dangerous to come up with a loose answer here. Many ADD meds are stimulants and as such have an effect on the heart. The other thing is that there are many different kinds of heart surgery done for completely different reasons (malfunctioning heartvalve, inflammation, e.g. endocarditis, arterial blockages, mending a hole in the heart muscle etc.).
My suggestion: First ask the pdoc what he would recommend for your son's specific kind of ADD, then doublecheck with the cardiologist to make sure that the recommended drug is safe for your son's specific heart condition.