I am never entirely sure exactly when the birthdays of my relatives occur either !
I miss my old Mum ringing me up the week before to remind me, and she has been gone 19 years. So, what I do is to send a card a few days in advance. Its not a bad idea in December anyway, as delivery starts to get a bit delayed due to the rush. If he gets it a day early, it won't be the end of the world !
OH'S son makes sure he is not late with any cards. Beginning of November he sent our Christmas card, also my birthday card, that isn't until end of December.
It niggles me a bit,but I've never said anything. I would love to know the reasoning behind it though.