3.5 months suddenly flipped body to the left

Ftm here. My 3.5months after coming back from ifc, we placed him on his changing mat on the bed, on his back, suddenly he rolled/flipped his whole body to his left and on to his tummy, however as his neck is still not strong, he’s not able to lift it to “tummy time” position. Do i let him be for a few minutes before turning his body back to the back position or just let him be ? And yes he’ll be highly supervised infront of us. Does this mean that he’ll learn to flip to tummy soon? Or not necessarily