Not only his memory, his whole brain function.
Who isn't interested in looking up information about their deceased mother or does not learn the last name of their friends? xD
I spent 6 years in college (2 more because of the pandemic). My university has 15,247 people, including professors, staff, undergraduate and graduate students. If I remeber the first name of 10 people in all these 6 years, it's a miracle. I probably remember the first and last names of 5 or 7 professors. Even when I was in college there were several times when I confused the name of a colleague with that of another who appeared in a photo or in another project and I had to be corrected by a third person.
Regarding MC's mother, I'll just say this: I never knew my father and no one in my family talks about it; all the people I could have turned to during all these years, when my family members avoided the question or gave incoherent answers in the hope that the subject would go away from my mind for good, have already died. It's a taboo subject even for the people who lives with me to the point I have to shut myself up about it to avoid fights inside my on house.
During my upbringing, my family, as well as MC's father, were extremely loving and effective in raising me well, within the limits of our economic situation, to the point where it almost didn't matter to know the truth. This only became an issue much later and something that I'm starting to lose hope in, as it will remain a mystery, unless the relatives who live with me decide to make some kind of explosive confession on their deathbeds.
So MC's situation is not as absurd as it seems...