You are a full grown man and you are this little boys father. You responsible for bringing him up. Do not abdicate this responsibility to cult followers. Tactfully and lovingly let your parents know that the indoctrination is not something you will passively permit. The boy should be your number one concern. He needs a father who will look out for him and protect him throughout his childhood. Be that father for him.
Tell your parents you love them dearly and treasure the time you spend with them. Tell them that you are taking full responsibility for seeing that your son grows up to be a good man that believes you can be good without god.