Okay, I have done some quick and dirty testing (cheating). You can add Greg via cheats like anyone else and edit him in CAS, etc. Doing so did not change his available interactions, so he probably has a hidden Greg trait that keeps him the same. Big plus that he doesn't seem as fragile as the sages. Also, I tried to test to see if the game would spawn another Greg and nothing so far, so that is also promising.
Going through his profile, he is just missing some basic information such as a last name (CAS will ask you to add one to save any changes), but otherwise has a full set up with relationships and werewolf perks. Just to see what would happen, I randomized up a little Gregette in CAS and everything was still normal back in live mode.
My tests were way too quick to confirm nothing was broken, but it definitely looks possible to force him into a family. I just don't think he was intended to be with his missing information.