When my son is asleep his ribcage draws in he has had sleep study and sats are normal what else could this be happen more when bunged up ?

Need more info: Very thin babies have little muscle and fat to hide their ribs, making "normal" appear "abnormal". Problems with either the heart or lungs (sometimes metabolism) can show with difficulties breathing or increased work of breathing. A careful history of what you see, a careful exam, and possibly labs/xrays are needed to identify if there is a problem and what it might be. Talk with his pediatrician!

See below: You did not mention your sons age which can affect the answer. Infants have softer or more compliant chest walls so when they work harder to breathe from various entities you can see the ribs get drawn in. Also known as retractions. Obstructive sleep apnea neuromuscular diseases or other breathing disorders can do this and i recommend evaluation by a pediatric specialist.