I was where you were at the same time. From what you are describing is Torticollis. I noticed when my daughter was 2 months old that she would only turn her head to the Right and she her head started to flatten on one side (plagiocephaly). We did take her to PT and she also recieved a DOC Band (aka helmet) and it ran us $3000 and no ins didnt pay. But it was worth every penny!
As far a Tummy time for kids with Torticollis this is something that is very unplesant for them. I never gave up on TT but only did it until she cried.
I tried this: Lay him on his back facing you and hold onto his little hands and slowly pull him up towards you. SLOWLY until he starts to lift his head up. Do this a few times a day and he will slow build up neck strength.
Also, sit him on your lap and hold his hands and help him balance in the sitting position. This also helps his neck muscles...
My baby girl was able to sit by herself by 5 months and had great neck strength. See, there other things you can do other than TT.
The main thing is for you to keep his head off of things. If you dont have one already buy a Exercauser and put a blanket around his body to help support him and let him play in there. Also Bumbo's are good as well. For the car seat buy the Boppy Noggin Nest Head Support this will help the continuation of the flattening see link: http://www.target.com/Boppy-Noggin-Nest-Head-Support/dp/B...
For PT you will need to do neck streches and try to make his whole world exist to his LEFT. PT is both your and the therapist's job. You will have to work with him on his neck streches throughout the day.
I would not wait another 2 months the sooner treatment is sought the better. If you have any questions please email me and I will be happy to discuss my expierences with you.