Not Over His Ex
No matter what we may say, there are times when our exes cross our minds. It depends on the relationship, of course. If the groom-to-be had a long-term relationship, was engaged or was even previously married, then that person was a huge part of his life for a significant length of time and he's not going to get over her very easily, regardless of who did the leaving. If he had children with her, then she's still in the picture, and that makes it even more difficult to move on.
One thing to consider is how quickly he started dating again after the breakup of his last relationship, as well as how quickly he got engaged. If it was a short amount of time, then there's more of a possibility that he isn't quite ready to commit again. That doesn't necessarily mean that he wants to get back with his ex. It could just mean that he needs more time, but that depends on both how he left his bride-to-be at the altar and how patient (and forgiving) she is.