Why would my patient's vocabulary suddenly increase?

That depends on how close to the end of life he is - my mother (almost completely non verbal) had an end of life rally and got very chatty just before her final decline.

I do not know what causes this other than a malfunction in the brain due to the type dementia.

I have known of two cases.
One was my DIL father with long term Parkinson’s and dementia. He was given a different drug if I remember correctly. He had been non verbal and just started talking. Seemed to be mostly current with who everyone was etc. It lasted just a few days and he was back to his previous state.

The other is also a Parkinson’s patient with dementia. Her daughter, my cousin, had started her on CBD oil. This was about 3 or 4 years ago. She had been nonverbal for over a year. She, like the other person, started talking and knew seemingly everything. She was sitting on the sofa with her daughter and said, whoops I’ve gotta go to the bathroom. Jumped up and went. She had been incontinent for a long time prior to this. She maintained her ability to speak for quiet awhile. Maybe a year. She had a bad fall and it left her again.

So, I would say, record him, visit with him, and know it could stop as soon as it started.

My cousin would call various grands for her mom to speak with on the phone. She would send me photos of a delighted mom talking on the phone. Cousin gave all the credit to CBD oil but it couldn’t bring her back after that fall.

It almost seems like an electrical short in the system that somehow breaks through the static and connects. A reminder to us that they are still in there.

The mystery of dementia remains a deep unsolved mystery indeed, and there are many more questions than answers. Anything we say would be a guess. It's a wonder, isn't it?

My mom is a stroke patient with no speech since 2011 and every year for about 2-3 months she will have high activity of very mild seizures then after about a week she will start saying new words. Now she finishes people's sentences and singing jingles. Her memory has also improved but it comes and goes. Doctors can't answer why but I really feel like the seizures are her wires trying to reconnect after getting damaged from her stroke. So much to learn about the brain

Something's gone ping! in his brain. Is he happy about this?

My grandmother did something like this. Or maybe I wasn't around enough during her limited verbal years to see what was really going on. But at some point after a couple months with her, 10+ years into dementia, she started -- and mostly continued until her death a year later -- responding to questions with complete ideas, saying bigger concepts and adding to them, and starting conversations at any time. She started spelling words out sometimes after she would say them, almost like she was reminding herself of previously known things. I don't know if this sounds similar to what you're seeing and I have no idea what caused my grandmother to do this. The only thing I can even imagine as any cause is that she did seem much happier and in much better mood at those times and after a few months of a different caregiver and myself in her house.

My father had a stroke and ran the gamut from making no sense at all to telling jokes and using 5 syllable words. Never figured it out.