i would continue to ask around, maybe someone will know this close to the date. what age is he? are the girls wearing formals or just nice dresses? if it is just nice dresses, then kaki pants with a white shirt and tie maybe. if they are wearing formals then i would definately do black slacks with a white shirt and tie, maybe even a jacket.
maybe ask someone who went last year. it depends on the area too, i know in small towns, like where i live, the boys usually do kakis and some in black pants, some in ties and some not and the girls just wear nice dresses. but my sister in law lives in houston and they go all out, it's like prom. they wear formals and rent tuxes. they even took a limo last year. so it really depends on what kind of homecoming dance.
hope this helps
good luck and hope your son has a wonderful time at his first homecoming dance, and be sure to take lots and lots of pictures.