What head support pillow would you recommend for flat head baby?

My baby is 4 months old and he has a very flat head developed from sleeping on the back too much. Since sleeping on the back is the recommended position to avoid SIDS, I am not sure it is good to let him sleep on the sides. Anyone knows would a head support pillow helped to correct the flat head? If yes, any particular pillow you can recommend? Thanks!

The pillow I recommend is MORE TUMMY TIME. No pillow, or sleep positioner is safe. At 4 months you may want to try a Bumbo seat for when he's awake too.

Baby bath pillow support cushion so comfortable