When should I be worried if baby has yet to flip? He is about 5 months

My boy is 5mths 11days old. He has not learn how to flip yet too. But he is turning his body to his side. I'm not worried as each child's pace of development is different. My friend's baby flip at 3mths. My boy doesnt flip yet but he wants to sit up. Isnt it a joke? Lol! Maybe u should wait till ur child is 6mths?