High school freshman boy & middle-school girl...Not okay?

My son is a freshman in high school. He's mature in most ways and is very smart, but likes two girls. The problem: they're both in middle school. One is a 7th grader & the other is an 8th grader. I don't think it's appropriate for a boy in high school to go out with a middle schooler. Am I right, or being too harsh?