Try rubbing his head with your palms in circular motions. Idk if it helps but I did that with my baby since birth and she also used flat head pillow (She was vacuumed out btw)… first thing everyone saw her was “why is her head so round”, so I guess it will more or less help? 😅 Alternatively let baby sleep in yaolan (cradle swing) or put a pillow behind and infront of him while you lay him on the side you want. (Please make sure you are there to supervise incase of SIDS.)