It's convenient that he has the fake grandparents are a backstory excuse, and I guess the rather far-fetched excuses are a running gag at this point. But I would like to see him just pull something like a "It's complicated and I'd rather not talk about it" at some point, it just seems like it would make more sense and be more realistic.
It's more realistic this way IMO. I'd do the same tbh, the lie and such, and the lie isn't harmful in any way, so why not. Doing the "mysterious man/boy" line felt rude to me, because IMO honesty shouldn't be paid with building a wall and distancing yourself, rather it should be paid with opening yourself a bit, even through some kind of white lie. You might argue that lying in front of such honesty is the rude one, and you're right, but words are just half of the equation, with such white lie MC can follows it up with fitting warm/positive attitude/body language. But with "mysterious" line, it will be hard to follow it up with positive attitude/body language or just brush it off, and the opposite side might think that MC been through "a lot", it might sour the already warm/positive atmosphere and conversation, hence why i think it's more rude to respond that way.
To me, truly honest and "mask-less" encounter/conversation is unrealistic in day-to-day occurrence or simple encounter/conversation. You'll put a "mask" to some degree, and it's more realistic that way. The difference would just whether you hide your true intentions or had ulterior motives or not.