My son had the same issues, right around the same age as your son. We started physical therapy right away and got him a helmet when he was about 5 months. We also tried some rolled towels on the affected side when he slept in the swing or bouncy seat to prevent his head from turning that direction (when we were around to watch, not at night). We also changed his position on the changing table and the placement of his crib aquarium so that he would have to turn his head the "good" way when looking at us/the crib aquarium. During tummy time, we put interesting toys on the unfavored side of his head so that he would make more of an effort to turn that way to look at them. I think doing all of these things, as well as the PT, helped with the torticollis.
We debated for a while about getting the helmet. We had heard that his head shape might correct itself without a helmet, but we were worried that it wouldn't, and then it would be too late (correcting the plagiocephaly via helmet does not work after a year of age). We also worried that the helmet would really bother him and somehow affect his development. But after weighing both choices, we decided to go ahead and get it. He wore it for about 3 months (some kids wear it for longer), and it didn't bother him at all. His head still has a bit of a flat spot but it is MUCH improved. We have no way of knowing if the flatness would have corrected itself without the helmet - maybe it would have. But what if it'd gotten worse? There were just too many unknowns, so getting the helmet made us feel a little more in control, and we were happy with our decision. I think you will get a lot of differing opinions about this - we got some very skeptical remarks when we decided to get the helmet - so you should, ultimately, do what you and your doctor think is best.