It seems as if they have adopted each other as 'fall back position'; if our present marriages/relationships don't work out we can simply pick up where we left off without having to be arsed to work out where our present relationships went wrong, or make an effort to get 'out there' to find new partners.
From the little that is known about the ex, spoilt and demanding has been mentioned and, as those words usually indicate a controlling personality who enjoys being the centre of attention, I suspect that it may be she who's gradually upped the ante in this correspondence.
She may be the type of woman who is of the opinion that she can have any man she wants and that includes her former conquests because she believes that, once they've fallen under her spell, they'll always compare other women to her and find them lacking.
To this extent the OP's dh may be being played like a fish, but his ex can only jerk the line while he's willing to stay on the end of the hook.
What is obvious is that the OP's dh is being disrespectful by implying to his ex that he may not be entirely happy in this marriage to the OP, and he needs to think very carefully about why he's acting in this manner.
Is he using the correspondence with his ex as a way of escaping reality? Indulging in flights of fantasy - including those over Italy? Is he reliving his youth in the hope that he can stave off the ageing process? Is he deeply insecure and using it to boost his ego, or to convince the OP that if she dumps him he's got another woman eager to get into bed with him, so to speak?
Effectively he's created a situation where the OP is, in the immortal words, one of three in her marriage. As that is one too many for most self-respecting women, he may find that continuing to intimately liaise with his ex erodes the OP's trust and love and he'll be stuck with his ex from hell either in fantasy or reality.
Personally, I'd be sorely tempted to book him a flight to Spain so he can see the windmill he's tilting at, and so that he come face to face with his ex's current partner.
Show him this thread OP so he can see that, in the opinion of the majority, he's being a plonker. As for missing his ex's dad - aw, my heart bleeds for him not. It's time for him to put away childish things, grow up, and start being a decent husband to you and a role model for his own progeny.