Sometimes admissions drops the ball on these match ups - or sometimes they’re dealing with too many moving parts to match every kid perfectly. I wouldn’t read too much into what classes he sat in on, but it may or may not be meaningful if his perception is that kids at that school were not engaged in class and that’s important to him. Just because 9th graders are in an introductory class shouldn’t reflect level of engagement. But if your view of this school is that there are many students on a completely different academic level, maybe it’s not the right fit.
He should ask the coach to connect him with athletes on the teams for his sports.