I’m not sure if my thoughts are going to be much use, but here goes. I’m Catholic and my wife is Protestant. However we were both Protestant when we married, so I know the sorts of issues that a Protestant / Catholic relationship might entail.
That said, is your fiance’s faith wavering in God, or the Catholic Church, or both? If it’s the Catholic Church, then his experience with your Protestant church might be having an effect. If that’s the case, then to be fair, you ought to attend mass as often as he attends your church. It doesn’t hurt to touch base with both sides of the Great Divide.
But if his faith is wavering in God, that’s a different issue, and a far more difficult one. You could find yourself married to an unbeliever if it keeps going.
In our case, my wife is the more consistent Christian in some ways, althougth she doesn’t go to my Catholic mass (I try to go to her church monthly for family reasons). That’s not because my faith is wavering, but I have reasons for being a bit cranky with God - in my opinion anyway. There’s no way though that I can deny He exists - I’ve had too many direct spiritual experiences for that.
So our situation is not the same as yours.
In a nutshell, if his faith is wavering in the Catholic Church, then you could help by learning a bit more about the Catholic Church and not just believing everything that Protestantism teaches about it, which is often plain wrong. But if he decided to go Protestant due to your influence, which is a possibility, then I’d be less concerned about that than I would about a loss of faith in God altogether.
But if his faith is wavering in God, then you’ve got a real problem. My suggestion, for what it’s worth, is that you ask God to give your fiance a real sense of His presence in some way or another.
He has never attended my Methodist church. I have attended Mass a few times so I do know something about his religious beliefs. I read lots of informative websites and links from CAF in particular. So it is not my belief that is causing him to wane from his own. I appreciate the advice and I will pray more with him. Thank you.