oh... and yeah- you may want to look into FMLA
Any employer can come up with an excuse to fire an employee. It happens everywhere. If, however, you do get terminated, keep all of your records and find a lawyer. I wish you the best of luck... and try not to worry too much. Everything will work out eventually.
I recently had a similar situation happen myself. I worked nights for five years and was very stable throughout that time period. Towards the end I became manic and couldn't sleep at all, my job was affected, etc. My pdoc had restricted me to an only dayshift work schedule and thinks I was hypomanic much of the time while working nights.
I had a great deal of trouble with my boss being workable with these restrictions. I could've grieved it and stayed working in that area, instead for my sanity I chose to take a day shift job within the same organization. It took me awhile to find the job I wanted and I was off work eating up fmla hours because I made the choice not to return to night shift.
I wish you the best, stay strong in your decisions and what you know is best for yourself. My pdoc says a normal sleep/wake cycle is one of the most important things for a bipolar person, in terms of our moods, stability, thoughts, etc.
I would say the simplest answer is probably the most likely. Your accomodation is a pain in the butt for him, perhaps other workers have complained it isn't fair. The easiest solution for him is if you cave and everyone follows the new system like good little ants. So he is being a jerk about it.
However you need these accomodations, so hold your ground. Because the reality is if you go crazy you will screw up your job and give them a reason to fire you. Keep notes. Record the date, time, who you talked to, and what they said, especially any praise you get. This way if it goes to HR you will have a record of it. The fact that you recently got praised makes it very very hard for them to fire you. They would have to pay you a big settlement and the Feds don't want the bad publicity of a wrongful dismissal trial.
All the evidence points to you being a good worker. The boss wouldn't say it if it wasn't true, it doesn't make sense. If they were trying to fire you they would be calling you into the office on some nonsense and writing you up.
Bottom line is he is held accountable if your work schedule can't be altered. It is his job and responsibility to ensure accomodations are met. I would insist to him that this is absolutely necessary and as much as you can understand how this is difficult for him, and you wish you could accomodate him, you can't. And if he schedules you for nights go into his office and tell him you can't work the shift. And keep notes. Assume the best, prepare for the worst.