Okay so since it sounds like he would be anxious w/too much going on, how about inviting the just family over?
You could always include 1 or 2 of his friends if you think he would feel
comfortable.
Serve his fave foods: (hot dogs, chips, cake & ice cream, pizza, candy, burgers etc. Whatever HE likes).
Have a pinata (or not if he would not be into it).
Show a movie he loves?
Themes?: the people from Whoville, pj party, dress up like baseball players, soccer players?
Make the entire day all about him: make his fave breakfast, decorate the kitchen w/streamers & ballons before he wakes up, make up a pretend helicopter cockpit inside a huge cardboard box complete w/guages, seats, etc., get little mini helicopers to decorate the table (Target $1 bins), make the living room a helicopter airport waiting room: chairs, magazines, make a fake coffee dispenser machine from cardboard box etc, get creative, have a pay phone in there, tables w/magazines. Call it "Your (enter your son's name here) airport) w/a big sign that says that, have popcorn in those little plastic white w/red stripes containers (@ Target) etc.
It will be so cute and fun.
Happy birthday to your son.