Personally, I think it was because it was no big thing for him.
My niece was born in Canada. Parents were on vacation in Banff when she went into early labor.
When they went home a week later, (pre-mandatory passport days) INS didn’t even ask them about the baby in arms.
She has US passport, no problems ever. She also did 2 years active duty after ROTC. Never thought about renouncing Canadian citizenship because it was never a problem for her.
has he said as much?