Generally you can get the flat spot to pop out, with some at home physical therapy, and re-positioning. Have they completely ruled out that he has Craniosynostosis or premature fusion of a skull suture? IF that was was ruled out, then its quite possible to go helmet-less. Especially if the flat spot is minimal. my oldest had a pushed in plate on the back of her head, they recommended that it be helmeted but with just some positioning, it filled out fine and she was spot free in a few months. She is 4 now and she was 5 months when the saw the spot.
My second daughter was actually born with Craniosynostosis. It was evident at birth but I was shot down by every doctor and ped, until I demanded a CT scan at 4 months and sure enough she had a 1mm pre-mature fusion of the Orbital suture. It required a complete brain/skull surgery and facial reconstruction at 6 months old. She HAD to wear a helmet for months to try to re-mold her skull after the surgery. She still has some degree of dis-alignment, strabismus of one eye, and torticullis as a result of her eye orbit being off. She is 2 year old now, cant see any flat spots or evidence other than a slightly lazy eye, and a huge scar on her head if you pull her hair into a pony tail. So I have done both and both work. It will depend on you and your commitment and research on alternatives. I will tell you DO YOUR HOMEWORK with your insurance. BE 100% sure what they will cover before you start, and be sure they dont require some PT before you sign to have it done, if they have some sub requirements, the doctors dont always tell you. We had a problem with our insurance covering the helmet for our daughter, and footed a 4 thousand dollar bill ourselves.
EDIT: of course they are going to say it wont get better on its own, because they are Cranial Technologies. They need the money. LOL However your baby is over 6 months and those plates are getting more solid, and falling more and more into place, it could be that the plate in question is already shifted to its position, and they feel with out a constant pressure applied it will not pop on its own. Though my daughter was 5 months going on 6 and it did it just fine, but that was the back of her head.