R.L.
How about asking him what he'd like to do? You can let him know money is tight and that a small budget would have to exist. At that age, just hanging out is something kids like to do- no matter what the occassion. An idea would be to go to a park (unless you have some good space in your yard) and let them have sports equipment and they will do the rest. Keep in mind too that many people that have birthday's near holidays don't like to do anything related to that holiday for their birthday.
I'm curious as to why you are already anticipating that kid's can't or won't come? What about expecting that they will show up. If his friend's have busy schedules, consider picking a couple dates and see which one would be most convenient for the most amount of kids and go with that date. You would be amazed how people can make things work to attend a friend's birthday.
It's possible that your son may not even want anything major and would prefer just a couple friends.
Sit back and relax- this whole thing could end up being quite easy.
Happy Birthday to your son.
R.