How can you tell if someone was abused as a child? And what can I do to help?

I do not know about the elementary school thing, children do tend to be very curious. If you believe that he was abused then i would say go with your personal instinct. Also the fact that his parents threw him out is a reinforcement to your thought of abuse. If you become too worried or afraid then i would say leave. But I would show him some support first... maybe ask about it. Recommending a counselor may also help.