Where there's smoke, there's fire. Just saying.
He doesn't let you spend much time around them.
if he's immature or sneaky, that's two big ones right there
I suspect the answer lies within the question.
If he has inappropriate girls as friends, that in itself is a sign of problems. Or maybe I misunderstood the question and you're asking what inappropriate friends are...
So you're saying he has an inappropriate relationship with these women? To me, its inappropriate if it makes you uncomfortable.