Now he finds out that he was living with a woman without the benefit of a valid Sacramental marriage.
Through no fault of his own. You’re forgetting one thing: a Catholic priest cannot celebrate a marriage that he knows will be invalid. Therefore, at the time of the ceremony, up through the time that the decree of nullity was issued, at least one of the spouses believed the marriage to be valid. The Church calls this, then, a
putative marriage.
Therefore, in good faith, he may have believed the marriage to have been valid. (Or – and we don’t need to discuss this possibility in the context of this particular case, please! – he might have been the one who was the reason that the marriage was invalid (through lack of due discretion, or simulation, or whatever the cause for the nullity was).)
So, either way, I don’t think you have a case: either he entered into it in good faith or he was the cause for the invalid marriage. Either way, he’s not harmed by the nullity.
Strictly speaking that means they were non-culpably fornicating if we were to get technical.
No. Strictly speaking, it was a ‘putative marriage’. No fornicating, non-culpably or otherwise.
it is clear the husband thought, and still does believe, that his first marriage was true and valid before God.
We don’t know that.
He may not be a Catholic for all we know?
Then what would a declaration by the Catholic Church mean to him? After all, if this is the case, then the state still holds the marriage to have been valid. That wouldn’t suffice for him?