C.A.
I really hope all these parents plan on staying with the kids. Or at least some of them. You really need SEVERAL adults to keep this one under control. Also have a timeout place and let all the parents know, in advance (so tonight??) that while their kids are in your home if something gets out of hand you'll be doing timeout (specify that you have 15 kids over and need to keep things from spiraling out of control). I expect that when my kids are with someone they get in trouble. If a parent has a problem with this then they don't need to bring their kid over.
Is there a movie your son has wanted really bad? I hate to say this but with that many kids you may consider a "theater" style party (I did this with my son and it worked really well). So do normal party stuff the first half. Cake, ice cream, presents and the second half open his new movie, and pass out goody bags for kiddos to snack on. Wait to do drinks till a little ways in when they may be getting squirmy and then give those out.
Definitely some structure. Another idea that we did was we bought little activities for them to put together. Head to Michaels and explain the situation. They probably have some kind of age-appropriate craft you can all do. Ours were these little foam car magnets. The kids loved them! I had a friend who bought pirate patch kits and those were a huge hit too.
You can run games with several adults helping. You will probably need to divide the group into two or three smaller groups and you can rotate them through the activities. It is seriously not unreasonable for some of these parents to help. I never just drop my kid and go at a party. When they are older (like 8 and up) I might.
Anyhow I hope something here helps a little. This is your home and your little guys party. This is about him and you are totally allowed to make sure it's the best no matter what. Best of luck!