Sports jacket and nice slacks might work, too. Is there any other boy attending that you can ask the parents what he's wearing? It seems like fashion is so different from one area to the next.
In the Northwest, a sports jacket and pressed (yes, really and truly ironed with a crease) is the look for semi-formal private parties. My friend's son in San Francisco always wore a suit and tie. Around here in Central Florida, it seems to be sports coat and slacks (but this is a retirement community with few boys although we do have a very good high school). Heck, I've even seen a shirt with tie and jeans be deemed appropriate.
Look at the super-sweet 16 parties on MTV. The "main players" are dressed up but most of the kids who are invited are much more casual unless the hostess has been very specific about what is to be worn.
