We don't know, the MC never said.Does the MC really not know his moms last name? Does it not make more sense that he in fact does but has just not reveled it in story?
Anyway, the name only matters if it's a family we know. Knowing that the game doesn't have and won't have incest, even between cousins, we can rule out all the families of the girls the MC is banging.
We also know that Lynnette's family was wealthy, which would leave only the Burgmeisters (Rusty) among the known names as a possibility.
If that's the case, it means hir father has always lied to him about his mother's name. Theoretically possible, but unlikely in my opinion.