What can I do if my OBGYN decided to stop treating me 2 months before my due date?

It's poor customer service, but legally, a doctor can accept or refuse different forms of insurance or payment on short notice.
So... your options at this point are basically practical remedies, not legal issues.
The first is to do as they suggested and find another doctor.  If this is the service you are getting from this office, you may want to strongly consider it.  I know that's a little a tough because you are seven months pregnant... but I will tell you from experience, sometimes switching doctors can be an accidental blessing.  I had to switch hospitals at the last minute with my first born--- and the doctor I was assigned just happened to be an expert in infant revival... which was literally a miracle when my son was born blue and with no heart beat.  Not a requirement, just a suggestion.
Your second option is to set up an appointment as a private pay with your doctor to discuss the situation.  You've been with him for seven years and you obviously have a rapport and comfort level with him.  Sometimes the ladies that work the front of these offices are not the kindest or most up to date on matters.  Talk to him and see if there is a possibility of his office accepting your insurance at some time in the near future.  My assistant did this and found out that the doctor and the insurance company were disputing fees..... but they resolved the dispute in a month or so... and she continued to see him.  If he is firm that he will not accept your insurance, see what options he has for you via private pay.  You are far enough along, that some of your expenses have already been paid.  Once you have this number, see what affect it will have on your taxes next year.  Depending on the amount, you may be able to take it as a deduction, lower your tax bill, and thereby offset the expenses of having to pay it in the first place.   If this is just not financially practical, discuss other doctors that your current doctor recommends.  It's at least a starting point of where to look.
Finally, call your insurance company and get their input.  They may have a way to continue coverage with this doctor since he's been the primary doctor at the start of coverage.  Some policies will not pay 'up front' for out of network doctors, but will reimburse you for a portion of the expenses.  Again... not ideal, but if you get reimbursed for a chunk of the expenses and you get to keep a doctor you are comfortable with... then it may be a viable option for you.
I know this is tough being seven months pregnant.  But, you still have a couple of options.  You may end up finding a doctor you appreciate even more than your current provider.  Keep an open mind and don't stress more than you need to for the sake of the baby.  I truly wish you the best.