Most Helpful Guy
It could be a rebound as a 5 year relationship can cause someone to be used to having a partner at all times. Also it could mean that he was already eyeing this girl prior to your breakup, which I hope not cause that sucks. Sometimes rebounds work and sometimes they don't. It's hard to tell now.
Most Helpful Girl
Its probably just a rebound. Or in a lot of cases people in long term relationships will start to think about being with someone else which in turn makes them, think their relationship "isn't working" the butterflies are gone or whatever you want to call it.
New relationships or sexual partners are exciting at first. Then he'll realize oh shit. Jacked up something that was real and this is just a fling.