Well, what I would have done (assuming I had the time to think about it, and respond accordingly) is say:
"Well, I don't think your mom would like that".
If he asked her, and she said yes, it would probably be fine.
However, I would have said "no" most likely, because one) it's a boy, and two) the mother probably wasn't hot. Sometimes I am looking for an adult GF, but only if they're hot. Motherhood is a big turn-off for me though.
Boys are worth zero risk, for me anyway! And I've already ended up with quite a few little boys to be friends with over the years... I don't need more.
Meanwhile, I am still girless.