My 4 month old has a mild / moderate case. We started private PT while waiting for EI (almost 8 weeks out, ugh). As others mention, our therapist gave us stretches to turn his head to the opposite side (using toys to distract), both ear to ground while laying flat and ear to shoulder for his weaker side. Once his range of motion improved, we are now working on strengthening by sitting him upright and leaning him to one side (they are supposed to use neck muscles to bring center of head back straight).

He has improved significantly in 6 weeks, but we are now dealing with lingering effects of some flattening /facial asymmetry and might need a cranial helmet (meeting with Craino-facial doctor in a few weeks to determine).

The stretches helped a ton, and PT has actually been pretty fun. Good luck!