Tummy time. Tummy time is the answer for most of these things especially if the flattening is symmetrical and on the back of the head. However see your doctor before we make those types of assumptions, he may have a little bit of trouble turning his head in both directions all the way. These are usually mild problems and stretching resolves it along with tummy time.
It's usually normal. Many vaginally-born newborns have some head "deformity" due to the proess of passing through the birth canal. The rapid growth of the brain and the softness of the skull usually correct the "deformity" within the first few months of life. Parents should seek consultation with their child's pediatrician, if the "deformity" persists, becomes worse and/or there is limitation of neck movement.