Mparks

I'm not a doctor but it sounds to me like you have both lymphedema & Trismus both very common side effects of radiation and neck dissections. I've had them both.
Click on the link here to OCF's explanations and read them
What is Trismus
Lymphedema-OCF
My entire medical team (RO,MO, ENT, Prothodontist & speech therapists) all warned me that after radiation, if I did not do jaw stretching exercises, I would not be able to open my mouth much. I have had great results using the Therabite device after my neck dissection and swelling, but just the basic jaw exercises worked the first time around. Why not call him and ask about stretching exercises?
I used the Fentanyl patches for a couple of months two separate times and never had any swelling nor have I read any OCF posts of the patches causing swelling.
I'm three years out from the first diagnosis and I still need to use my Therabite or my mouth closes up. I really hope your doctor is right about six months clearing it all up, but again, I'd call him or visit him with OCF printouts and ask about stretching exercises and lymph massage.
Charm