By your choice of wording, I presume that your son’s father is not your husband. Whether you can help him or not depends on whether you have sufficient relationship with him that he will listen to you. In the meantime though, your primary concern must be your son. Since your son’s father used to be Catholic, he certainly agreed to raising your son Catholic, so he should not be undermining your son’s Catholic faith. If you share custody of your son with him, I recommend expressing to him your desire that he stop taking your son to church with him when your son visits – or, at the very least, that he stop asking your son to receive communion in his church. If he will not agree, you will need to speak with a lawyer to see what can be done to re-examine your son’s custody arrangements. If your son’s father will not stop indoctrinating your son in another religion against your will, you may need a court to determine that the original agreement to raise your son Catholic is legally binding.