^^ I agree with Olivia. That’s probably the best option for avoiding a helmet.
Turn him to the other side. My son used to like sleeping on his side too so I would just alternate sides when I could. His head ended up turning out round.
Maybe it’s torticollis. Look up the head & neck stretches for torticollis. They had me do the stretches when my son had a flat side of his head from favoring a side.
& even if your baby doesn’t have it, stretching his neck gently won’t do any harm
I agree with the physical therapy recommendation. We’ve been doing it with our son and it’s helped a ton.
Definitely go to PT, make sure to find someone who specializes in torticollis and plagiocephaly. My son Had both, started PT and got a helmet at 3 months and graduated at 6months. He’s now 2.5 and you would never know his head had any imperfections.
have you seen a physical therapist? I would ask for a referral to see a pt