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

On generic terms babies usually start to do mini push ups at about 3 months of age. Afterwich thry start to flip at about 5 or 6 months. However it varies to each individual there are babies who do not roll over nor flip as well. Babies develop skills differently, some more quickly then others. I would say by 6 months old if baby has not showed any signs of flipping please bring it up to your pd on your next visit.