Nobody moves on and finds someone new that quickly.
(Of course, if he cheated on you with another woman and started dating her right after you broke up, it might be more serious than a rebound.) Other than that situation, the easy rule of thumb is that the longer he waited, the less likely it is to be a rebound relationship.
When you've shared your life with someone, it can feel excruciating when you learn he's now enjoying his favorite sushi restaurant with another person.
It's important to move past this feeling, as otherwise you risk becoming an amateur detective who is focused on her ex's life to the detriment of her own.
Fortunately, you can still give it another try before completely giving up hope.