I wrote a whole post and lost it. But I will leave you with this.
My parents split at 17 and I was miserable when it was the 3 of us. it was pretty uncomfortable for me.
This is why I suggest you ask him what he wants. You make think he wants a family dinner, but it might actually make him uncomfortable. Leave the question open ended.
"what would you like to do for your birthday?" His answer will tell you a lot.
