I have been privy to the inner workings of enough nursing homes to know that while there are doctors, nurses and caregivers on every level who are wonderful, attentive and compassionate people, there are unfortunately many more who are only in the business for a paycheck. Those people can become very impatient with any resident who requires what they consider too much attention, or whose behaviors strike them as annoying or difficult. Their responses to that can range from neglect to verbal abuse, from passive-aggressive physical abuse to simply drugging them into a vegetative stupor.
Its very important that anyone who has to leave an impaired loved one in the care of others remain constantly vigilant. I personally know of an incident in which a doctor in my community at a private Alzheimer's care unit, loudly berated an 80 year old man with Alzheimer's disease and OCD, telling him he was worthless and unimportant and refusing to allow him to use the phone to call one of his children. That's flat-out illegal - you cannot prevent a patient from contacting their family, but he does these things with impunity because he figures an Alzheimer's patient won't remember the incident long enough to report him. A more compassionate individual in that industry once told me "They may not remember your name from one day to the next, but they remember how you make them feel". The sad thing is, once a resident has been intimidated (or drugged) into silence, that facility which should be their home, becomes a prison.
Perhaps you might try removing him from that facility for a while - an afternoon, or better, a weekend, so you can see whether his demeanor improves. He might be more communicative outside that environment. There is clearly something going on to cause such a dramatic shift in his behavior. Best wishes, and keep us posted.