@GemimaMama - Yuck! Yeah, it's totally common. My belly looks a little caved in down the middle, too. It's called abdominal separation or diastasis recti. But there are exercises you can do to try to correct it.
It can also be repaired surgically, but I would make that my last resort. You don't say how long ago your son was born, but it takes a long time for the body to get back to anything close to its old shape. You're looking for strength exercises like planks, etc.--crunches will not be as helpful for this problem.