My son has very mild (barely notable) aortic stenosis and mild aortic leak. how bad is it?

Right now it's ok: Mild aortic stenosis and leak typically do not need anything done surgically. But it will need to be followed up with echo's on a routine basis. This could worsen as he grows up and may eventually need surgery. Your cardiologist will have the best info for you. They have seen the echo and can give you the best estimate.

Mild: Mild aortic valve disease is just that, mild, but assuming your son is fairly young, it will likely progress over time and should be followed closely.

I presumed: He is under the care of a cardiologist already. It is best that you addressed these questions / concerns with the specialist as he has all the test results and can give you a more meaningful answer pertinent to your son's case as far as restrictions/ activities goes and preventive measures.