Once he gets to Jerusalem, he's able to get the tribes of Judah and Benjamin to swear their allegiance to him. Two tribes out of twelve ain't bad. That's like a 17% retention rate, right?
Rehoboam starts to raise an army, but God advises the Southern Kingdom of Judah not to mess with Northern Israel. It's not smart to argue with God, so they halt their plans for an invasion.
So King Rehoboam rules over Jerusalem and some of the surrounding cities after that. His main priority is building up defenses in any city that might be attacked by the northerners.
It just so happens that all the priests and Levites (who do some of their most important work in the Temple in Jerusalem) are also willing to side with Rehoboam. They don't like Jeroboam much anyway because he went and chose all new priests once he became the king of the north.
Rehoboam gets married a couple times and has a whole bunch of sons and daughters. He plans on making his son, Abijah, the king of Judah eventually. The future's looking bright for now.