Who says he doesn't know his mom's surname? It hasn't been told to us because it's not interesting. We don't even know the mc's surname, does that mean he doesn't know that either?
Maniacs have been creating theories about Lynette's heritage because we don't know her surname, but nothing in the game points to there being some mysterious heritage, it's just wild, unfounded speculation. We know she's from a rich family who work in construction. There may be more details about that family, but none of it needs to hinge on her surname.
Now, if it turns out Lynette's surname is of great significance and part of a "big reveal", I'll be pissed, because the mc would of course know his own mom's surname! Supposedly Neil isn't the best storyteller, he fucks up punchlines and stuff, but he doesn't have Alzheimer's, he knows his wife's fucking surname...