So sorry about your situation, but glad to see the prevailing advice is to not beat yourself up about someone else's mistake (the son.) You've done nothing but right by the mom, it sounds.
As for next year and the "mark your calendar" approach, I would never never have a cake/BC ready without an order in place. (2 weeks notice.) I suppose the plan with that would be to satisfy the son's future-last-minute order? Or to contact the son asking him if he's ready to order a cake for his mom? (complete with follow-up until he's ready?) Uh, no way. I would never keep reinforcing the poor-planning habit. Imagine if you had other clients, too, who started expecting that last-minute orders are perfectly fine with you and you started prepping cakes for orders that haven't been made? That's not a business, it's a birthday cake charity. JMHO
If you don't want to totally alienate the son, gently remind him to set a Google calendar reminder 3 weeks before his mom's b'day that says "order cake for mum." Mention that it works in your household, etc.
Good luck
-j
"What a brave corporate logo!"
"What a brave corporate logo!"