I heard this passage from the film about St. Thomas More, “A Man for All Seasons”, referred to in a homily recently:
*Virtually all the lines in A Man for All Seasons are timeless, but a few stand out. There is the scene in which prisoner More is urged by his longtime friend, the Duke of Norfolk, to sign the oath, expressing approval of the king’s divorce and remarriage and proclaiming the King of England the supreme head of the church in England. After all, all of the nobility and all the bishops but one (Blessed John Fisher) had signed that very oath.
“Look at these names, Thomas,” Norfolk pleaded. “Won’t you come with us, for fellowship?”
“And when you have gone to Heaven for following your conscience,” More replies, “and I have gone to Hell for not following mine, will you come with me — for fellowship?”*
I think that if you do what you can to form their consciences correctly, you may also defend the right of your wife and children to follow their consciences. What that does and does not allow you to do may be something you need to play by ear…or rather, something you may have to do by playing by conscience. Inform yourself of the rules, but also listen to what your informed conscience tells you with regards to the actual effects on your particular family as you make these decisions.
If your oldest son is old enough to express a wish about it, then it probably has to be his decision whether or not to convert to Catholicism. Parents can only decide on behalf of their children until their children are old enough to have a reasoned opinion of his or her own. What exact age that is depends on the child, but I don’t think any priest will baptise a child who is old enough to repeatedly protest *“I AM A MORMON! I DON’T WANT TO DO THIS!!”. *Certainly you can allow your spouse and a child who has reached the age of reason to attend another church, because you have not made marriage promises in which you accepted a duty to attempt to bring your children up Catholic. Your wife, being Catholic, had a duty to be married according to Catholic canon law and to make those promises on behalf of her children, but that is water under the bridge now. Being the child of a baptised Catholic, your child had the right to be baptised and raised Catholic, but again: Water under the bridge. That does not mean that you, as a Catholic parent, cannot require that he learn about your faith. That, like your decision that he has to learn math whether he likes it or not or to learn history whether he agrees with the teacher or not, is up to his parents, and will be for a long time.
If your spouse decides to come back to the Catholic Church, your son’s religious rights might become an issue, but that kind of difficulty is a poor reason to follow what you believe to be lacking truth instead of what you believe to be the truth. That applies to both you and your wife. You have to follow what your conscience tells you is true, and face the trials that this brings on. If you and your wife promised to bring your son up in the Mormon faith and he (being of an age that this is his decision) still wants to practice, you may find yourselves duty-bound to see to it that he is allowed to do that. When you allowed him to be baptised a Mormon, after all, you gave him the rights of a member of the LDS church.
As for what you may do to honor the promises you made when your son became a Mormon, you and then you and your wife ought to see the pastor of the parish where you wish to go through RCIA, and talk about that. That is where you will inform your conscience about the various moral principles that you will be weighing as you navigate these issues. As far as I know, the presence of a Catholic spouse who refuses to come back to the Church is not an impediment to the baptism of a non-Catholic. Still, talk to the pastor at your Catholic church. He’ll be able to lead you through the ins and outs of this.
you’ve had some great answers to your questions, so I’ll just lend my support and let you know I’m praying for you