What is your birthday budget? $5? $10?
Does he have any friends you can invite over? He can decorate the cake and you can make them a frozen pizza.
I guess what I'm asking is, what do you have to work with?
He could have a couple of friends over for a movie night! You could pop a pizza in the oven, make some popcorn and rent a movie! Very cheap and I always loved those nights when I was a kid!!! (along with some chocolate too...or whatever his favorite snack is!)