They've dated off and on..... I was hoping to hear someone say slim to no chance
How reliable is this female? Could she be lying or trying to trick him? Are they in a relationship or was it just a one off? If he has any doubts about it then he can always demand a paternity test. He should keep that in mind if anything about this is suspicious to him. It sounds like he's a responsible guy so it would be a shame if this girl was taking advantage of that.
If it came off during, inside of her then yes it's possible.