Flat head

FTM and my 3mo old had a flat head. I’m doing the best I can to help prevent it from getting worse or to avoid pressure. I rotate his head to each side when he goes to sleep, but still ends up laying flat on the back of his head. He hatessss tummy time but I still have him do it at least 8-10x a day, he lasts anywhere from 1-3 mins. Even during the day while he naps, I put him on my chest so he is off his head but still it’s so flat 😭 what else can I do?? Will it eventually fill out once he can fully hold his head up and sit up?

( I do see the pediatrician, not until February tho 😭)