I would celebrate the weekend before or maybe the day after on the 26th. That way family & friends can attend and it not interfere with anyone's Christmas plans. I have a couple of cousins that were born on Dec 24th and 25th and that's what they always did.
Congrats on your first little one! I just had my second three months ago, she's an angel! I'm sure your son will love whatever you plan to do! Have fun with it!