That's a great lesson to teach your son. When you are a good person, good usually comes back to you too, but that shouldn't be the motivation. You were very clear that you were not looking to get anything in return.
My father brought me up that way too. We had elderly neighbors and he taught me to shovel for them and do random acts of kindness.
It sounds like you are bringing your son up to be a good man.
RE: Good Deeds Pay Forward