OT- children with birthdays near Christmas

Bowser • 🌈🌈 due March 2016
How do you make it work for you? 
My sons birthday is a week before Christmas. He is 3 this year and understands already. That you get presents for Chrismas and your birthday. He is terrified off santa so has asked for no present from him lol.
 
My family has decided to just get him one present for the both. 
Am I being silly by thinking this is not fair on him? I understand he is only 3 but I know everyone is going to do this in the future if I allow it from now.
Could be my hormones too but I don't see why it is fair for me to have to buy 2 presents for their children even if they are not near Christmas.