M.R.
You are a lucky Mom, and he is a lucky boy; this is not really a dilemma at all. One party is enough for any five year old, and what to do on his actual birthday is not such a big deal. Send a treat to school, make his favorite meal for supper, get a cake, give him a presant, and talk about what fun his party is going to be. You dont have to, and probably shouldn't, invite every single person he knows, school included, to every party he ever has, it just insn't neccessary. You can have a big party one year, and a small one the next, and you don't even need a reason.
Special is really what happens every single day, especially if this is the only problem you have. He is going to be 5, save something for his graduation and wedding!
M.