Flat spot

So my son is five months old and he has a pretty bad flat spot :/ I suspect from sleeping in the rock n play with the hard plastic underneath for 12 hours at night. I have since put a blanket under the head rest to give it some more cushion after I realized. I hope to move him to his crib soon. Anyways, I asked the pediatrician at his 4 month check up and they assured me that it would soon correct itself. Well he sleeps on me during the day and he's starting to sit up a little and I hold him a lot and do some tummy time but I really haven't seen much of an improvement. Im starting to get worried that as he gets older it won't fix itself because the bones are getting harder and I don't want him to have to wear a helmet. Does anyone have any experience with this? Did your 6 month olds or even older child's flat spots correct themselves as your child got older?