"Well, it might be a relief," he replies with a smile of amiable ambiguity. Although, as far as the diapers go, after reacting so badly to a de-aged Iago, he'd probably have to go play in traffic or something, just to avoid hypocrisy.
The thing about Dynast, and probably half the reason he was till frozen up North when Xel left, is that he Does Not Share. The emotions he prefers and creates barely register as food at all to other mazoku, showing up on their radars only as a heavy, cloaking weight that Metallium, at least, always wants to claw through, and the light, unpleasant aftertaste of ashes.
Xel clutches the ember of a promise to his grey heart, for warmth. It's not that Gaav's familiar hands make him flinch as strange ones do, not at all, but if his person doesn't care about this, then how can Xel care about anything? But he doesn't need to test, does he? He can't need to. That's all there is to it. Iago is Zelas-sama's at least as much as Gaav-dono's, so he might be sly about it (in fact, Xel rather hopes he will), he might make compromises, but he won't... he won't.
But Gaav has had Xel's enthusiasm in his hands before, so a simple no won't do. This way, by claiming the right to defend Iago and his decisions, he's made Iago the player and himself the board, rather than letting him become an obstacle to Gaav by saying something like ask for something else, I'm a one-man dog, rather than letting him become a barrier whose removal will open doors. So he doesn't tell Gaav he's asked two gold for one; he doesn't give Gaav that leverage. He just sits serenely, like carved willow, and holds onto faith with both hands.