It is normal affair baby was not born premature or if the baby weight right now is less than 4 kgs then only you need to see your pediatrician otherwise it is normal baby will roll over slowly some babies take around 7 months until and unless their neck muscles are strong the back muscles are strong for the flip try more of tummy time with a baby to improve the neck muscles

Hi Anju did you try putting toys infront of him? Please consult his Paediatrician.