Hi M.,
First off, please take a deep breath and know that some of us go through this every year, so now look what you have missed for 6 years!! LOL!! No, it is tough to decide whom and who not, but I would go w/what is the maximum of friends he can invite and what will your budget allow?
I would only invite those friends who are close to him and that you associate w/their parents as well. My son has tons of friends too, but I just narrow it down to the ones he actually associates with on a daily basis and the ones whose parents I am friends with as well. This does make it easier and I know from my son's MDO he attends, I have already ask the teachers NOT TO put any invitations to b-day parties from his little friends at school in his backpack. That is a rule there, about inviting friends from school, if you invite one, you have to invite all of them. So, if you do want a friend, send them an invitation through the mail only, not at school. How you obtain their address would be either through their school directory or ask the parent.
First of all, I don't know the parents very well, we only greet each other when we drop off or pick up our kids from school. I don't associate w/them at any other time and I provide a small celebration for all of them when it is his b-day (actual day if he is in school) w/happy cakes for all of kids to share. This saves me a lot of trouble and I don't make other parents feel uncomfortable by inviting them to b-day parties of folks who I don't know and also the awkardness of having to buy a present for some kid who I don't know what they like or even want.
Again, other folks will differ w/me, but to top of all the above, we also have to contend w/siblings as well. Whether they should be invited or not, again especially when the place you have the party at allows only so many kids!! Another thing you will run into is some people just WON'T RSVP!! Unbelievable, but they just don't. Big pet-peeve of mine, but I have just learnt to either phone (if possible) or email them and remind them to RSVP. It makes it hard to plan if you don't know how many kids are coming or Not coming.
Believe it or not, my son's 4th b-day is coming up in June and I have already reserved and paid for a place because I hate waiting till the late minute to do this type of planning! I am so glad we only have to do this once a year!!
Good Luck and try and have fun!
G. B.