C.P.
My granddaughter had this problem. You may not need the helmet, yet, as she did not. However, you need to lie him side to side only. Avoid the back as much as possible, as this is what is causing it. I do not recommend tummy sleeping. Research supports this may be one of the causes of SIDS related deaths. Most pediatricians recommend not sleeping infants on their backs. Use support behind his back to maintain him on his side as much as possible (they sell supports to prop them with or use a pillow or rolled blanket). My granddaughter's flat head became better as she approached her 8th month in age. This is the most important thing to do at this time to avoid flat head shaping issues. My granddaughter's flat head reshaped itself just from not allowing her to sleep on her back anymore. I would use the helmet as a last result as it may not be extremely comfortable and he may refuse to wear it. It could become a bedtime struggle for him and mom.