I don't think he is too young for this type of party but here are some tips I've learned over the years with sleepover parties:
1)limit the number of guests to about 6 for this age group
2)even numbers work better than odd so that even if kids pair off to play, it's less likely that someone will be left out
3)save the movie until the end of the night as settle in time before lights out and be sure to choose something that all the parents of the other children attending would approve of as their home rules may be different from yours
4)plan a little scheduled/structured time and some free time for play
5)boys this age usually enjoy scavenger hunts and that is something you can do very inexpensively indoors or out in the yard if weather permits
6)go into this with as much patience as you can muster, a watchful eye (even when you are trying to let them have a little space), and the understanding that it will take them awhile to settle in and fall asleep
7) another fun thing to do is an "indoor" campout - you can set up a small tent if available, make microwavable smores or make trailmix for a healthier option
8) even boys like to do crafts so maybe think of something they can be creative with as a back up plan in case of boredom or if you feel a supervised activity is needed
9) Just my opinion, but I would keep the wii out of the mix until he's a little older (save it for one on one playdates instead) as only a few can play at a time anyway and the rest will be sitting there waiting...of course you know your son and his friends best and can make that determination...it's just a very expensive investment and kids get excitable in groups which can lead to accidents
10) when everyone arrives, go over a few simple but firm rules for your household - you want this to be fun! so don't go overboard with expectations but be sure to fill them in on your key rules (this works well to do while they are eating the pizza as they can listen while they chew - haha)
11) offer a 1/2 sleepover for those boys that may want to celebrate but just aren't comfortable sleeping away from home yet...they could stay for some fun and be picked up around 10pm or so
GOOD LUCK! HAVE FUN AND HAPPY BIRTHDAY TO YOUR LITTLE GUY!