The first thing I would do is confirm with your son that he, and his friends, are indeed planning to go to prom. Our school has a combined junior/senior prom as well. Lots of juniors don't attend. It's very expensive and the kids tend to want to focus on creating one big memorable experience at the end of high school.
The junior boys who do go tend to be in relationships--my DDs BF almost didn't ask her to prom their junior year--even though they went to every other HS dance together. And she was okay with that, although she was thrilled when he did finally ask her. (If DS is in a relationship, he ought to take his GF to prom.) Most of her friends didn't go that year.
I know that things may be different at your son's school, but before you go changing trip plans, I'd check out whether or not he's going for sure. Good luck.