No need to worry too much. He will learn it. However, you can teach him by placing him on the bed or play mat and position his leg to the flipping position then you use your hand and arm to turn his upper body and head. You can google on how to teach baby to flip
Yup more tummy time helps and look up for exercises on youtube to strengthen his muscles. I started since my baby was less than a mth old and ard 2.5 mths he managed to flip frm back to front.
Don't worry too much babies will reach their milestones when it comes,
Just do more tummy time