When my son tightens his stomach it flattens out on the sides& a vertical line up the middle pops up he is 6 mo. what is wrong?

Diastasis recti,: Separation of the rectus abdominus muscle, looks like a ridge down the middle of the belly from the bottom of the breastbone to the belly button when an infant tries to sit up. You can feel the muscle edges when he's on his back & relaxed. It goes away as the muscles grow & requires no treatment unless there is a hernia between the right & left sides with redness, pain & vomiting.

Benign: Usually this a mild immaturity of the midline abdominal muscle called the rectus. The pediatrician can tell. Usually disappears with age.