Which I find odd... Thordan VII summons a Primal, prompting Nidhogg to get serious, and Midgardsormr joins the song, but he does nothing for Tiamat and Bahamut? Seems unlikely to me, perhaps Tiamat brought great shame upon the family so he buried his head, but he certainly didn't give that impression when we met her...
I'd imagine the difference is whatever he was guarding... Pre-XIV the "seal" was intact and he wasn't allowed to leave the area at all; Crystal Tower was built next door to mock him perhaps. Post-XIV, he destroyed the "seal" (yes, Middy did, he crashed the Agrius into it, he at least takes part of the blame IMO) and is thus no longer bound to the lake, able to join his son until we show up and he instead follows us.