flat head baby

My lo is 3 month old and has a flat head. He likes turn to one side, but he can turn other side too. But has preferred side which is left. I have bought donut pillow..but seems doesnt work. I will go see doc in NUH but the appt in Aug. I dont want him to wear the helmet . What else can i do? Beside tummy time. Worried mum. 😥