I am a US citizen, can I submit a petition here for my brother who is currently living inside U.S. and going to college as an international student? Does he need to leave U.S. later and go back and wait in the origin country where he came from? Thanks!

You can petition for a relative who is currently in the US.  Since a visa number will not be readily available for your brother, it will be many years before he can immigrate based on an approved petition.  If, at the time a visa number becomes available, your brother is in the US, he can apply to adjust his status rather than process at a consulate abroad.  Chances that he will still be an international student many years from now when a visa number is available; however, are not great, so your brother will likely process through a consulate anyway.