My teenagers are girls, they are a whole lot different than boys.
Put him at a table where he might actually be a little shy, say if there are any teenage girls he's never met, that might intimidate him enough to behave. If not I think you should just split the teenagers up and scattter them at different tables, maybe two and two. Talk to him and tell him how important this day is to you, he may surprise you, and be the perfect little gentleman. Another option can you make him responsible for something, give him a camera and ask him to take pictures, run the music, help the guests find their seats. Keep him busy, make him feel important, and I think you will find he steps up and does you proud.
Kelly~