flat head

hi dear parents. so we went on our 4 month check-up. I think my lo having flat head on his back. and I was talking about it with his pediatrician. but looks she is not worry about it. she said I have seen worse situations. she said wait until his 6 months check up and see how he is doing. I was reading on forums that after 6 months repositioning of head will not help. I'm so worry. is that true that after 6 months repositioning will not help? any advice