Infant head shape. Possible in detail.

My newborn baby boy is 2 months old . From birth his head is little bit flat from one side (behind the ear ) means looks not in round shape . I regularly changes his head position from left to right , right to left . But whenever i keep his heads flat side in upper/relax position he turn his head again towards flat position . There is no problem in neck because Whenever he is awaken he turns hi head freely in normal all direction without any effort but while sleeping he keeps his heads flat side more towards mattress side . Please tell me what i do now so that his head become round and while sleeping he keeps his Head straight or the flat side upwards. Is i use any type of U shape pillow or what to do...! Please reply to my problem as soon as possible in detail .