I may be able to offer perspective here. I will be marrying a baptized Catholic outside of the church in just a few months, although we are both atheists. It will be an invalid marriage.
I would not go through a convalidation. I would not cooperate in any way. The desire alone to do that would cause a huge problem in our marriage. Threatening to withhold sex would probably end it.
FH and I are in love, planning our life, getting married in front of all of our friends and family. Not only did we write our own vows we wrote the whole ceremony. We have both made it clear that this is what we want more than anything. If we went through with this and he came to me several years later saying that he needed to make our marriage “real” and have it recognized by the “right” people I would feel like he was cheapening everything we said, everything we went through, and the vows that we had lived out for the last 10 years. Nope, I would probably suggest to him that if he feels like our marriage isn’t right or “valid” that we need to see a marriage counselor.
If he told me that he would be choosing the eucharist over being a husband to me as much as it would tear my heart and our lives in to pieces I think that it would be best for me to know that I wasn’t his first priority and that he found something he loves more. Yeah, we would probably get a divorce.
While I understand what you are saying, I think it is a little more complicated than just that. If your fiance were to later change his mind about the Church and you were to refuse to allow him to live in accordance with his beliefs then you would be rejecting him and throwing him out simply because his way of looking at the world changed. This is a complicated two way street here. In the one case the non-Catholic could feel rejected and hurt, as though the Catholic no longer loved them enough, while for the Catholic it might truly be a matter, not of loving his spouse any less, but rather of realizing that he has to keep his internal integrity and be true to himself and that his unchanged love for his spouse must be expressed in a way which does not go against who he is as a person. People always changed. When one marries they know that they are going to marry a changing person, and yet they still promise to love that person unconditionally and to live for that person, even when that person changes. So the situation could be looked at from the other side, if a spouse changes and wishes to return to the catholic faith is it not the non-catholic spouse who would be rejecting the catholic if they failed to allow them to do this? I guess it all kind of depends on how you view marriage and what you mean by your vows when you say them. After having seen the way that my mother lived out
her vows even after she had been abused and cheated on there is no way I could marry someone who would ever mean anything less than an actual
absolute commitment no matter what, despite how I might change and I fully intend to do the same. And don’t worry, my mother did not let my father walk all over her. When it became obvious that it was necessary she moved out and took all of us kids with her, moved far far away and raised us by herself. But she never spoke angrily or with resentment about our father to us. She just let us know that he had taken a wrong path in life and that we should pray for him. She would never dream of looking for another man to comfort her or take his place. She had made a truly unconditional commitment and stood by it even in the worst of situations. And watching her has helped me see that actual unconditional commitment to another is truly possible. It is truly possible and it is truly what we all deserve. To divorce someone simply because their philosophy of the world had changed to me definitely falls short of the kind of love and comitment demanded by the unconditional vows of marriage. And please understand that I am not trying to judge you personally in any way shape or form by saying this, but just trying to give you another point of view from which to think about it all. I know that however difficult it would be to deal with being married to someone who changed their beliefs drastically, because of my vows to them I would still stay with them, unless it led to a form of abuse, and even then I would refuse to live with any other man because I intend to truly mean my vows of commiting myself unconditionally to my husband, no matter where that leads me. I hope for the best, and plan for the best, choosing my husband carefully, but if it were to fall apart I will always strive to live up to my mother’s inspiring example of what true comitment and love really is. That is the kind of comitment the Church asks of spouses in marriage, the kind of love we believe is possible because of our belief in a God who has shown us the way through his own unconditional comitment and display of love for us on the cross. And my mother is absolute concrete proof that this is not just some unreachable goal, it is possible for all of us, if we just truly aim for it and set our hearts on it. I understand that this comes from a different worldview, but I cannot imagine a more beautiful life then two people living united to each other through such a mutual unconditional comitment of self.
oh, gosh… I’m kind of just rambling now. so think about it if you wish, or completely ignore it, I don’t care. But please do understand that I do not mean any of this personally.
Friendly Neighborhood Methodist around here, but I think that some of us (including me!) should be holding this gentleman in our prayers. Along with his wife & her family.
painful place to be in, & maybe if enough of us 
pray about it, the hearts of the wife (& her family) will be softened to realize that his spiritual needs are not going to be met unless & until he is able to return to the practice of the faith that he truly believes in (& is a part of).