Sorry, but no. Under the immigration laws at the time, as the minor child of a US citizen, he was eligible for naturalization. Hence there was no need to fabricate anything if he indeed was born in Canada, as you speculate.
That is a good point. The fault in it, if it has one, is that it requires the Grandmother to know that piece of information.
People often act on what they BELIEVE to be true, not necessarily on what is actually true. I wouldn't expect average citizens to be aware of naturalization law till they actually have need to deal with it. For all she knew, the Canadians might Claim him, or Perhaps that "British Nationality Act of 1948" might make him the property of England or something. I see people even NOW presuming that, and THEY have access to far more information than she likely did.
Americans nowadays are pretty loud about that "Born on the soil" thing. The 14th Amendment has skewed a lot of people's understanding towards the "You have to be born in America" mindset. It would not surprise me to discover that she had that mindset in 1961.
Actually, now that I have thought about it further, a lot of people have pointed out that Stanley Ann was too young to convey citizenship because she hadn't spent sufficient time in the USA after her 14th birthday. So maybe in fact he Wouldn't be covered by then existing naturalization law.
What about this aspect of it? Is there a minimum age/residency requirement?