How can I help him out if his visa expired and he wants to go back home to visit his family?

There is no visa category by which one may sponsor someone for immigration benefits as a boyfriend or girlfriend. If you are a U.S. citizen and you and he become married, then you could sponsor him in an adjustment of status application process in the Immediate Relative visa category, by which he could become a Lawful Permanent Resident (get a "Green Card"), and in that status he would be able to travel abroad to visit his family and then return to the U.S. This is true notwithstanding that he originally entered the U.S. lawfully and with inspection, but then overstayed his visa.