This may not be of any help, but .... if it was your close Grandmother (known all your life) .... the situation would be even worse. I don't mean to minimize this at all - but maybe your boyfriend needs some evening breaks, as well as, you!
Is there a chance that the two of you could find someone (Grandma would trust) to come in two nights a week ... so that the two of you could get out of the house at least one night a week. Call it a date night or whatever. You two need breaks - even if your BF doesn't appear to show it.
My Mother just passed about 6-weeks ago, after being in an Assisted Living Home for Dementia for 16-months. Before that, she lived at her own home and had 24/7 caregivers for 15-months. And before and during that timeframe, I called her daily for 16-years and went to her house (40-mile round-trip) for about three years while paying for all of her bills.
I did not live with her during those timeframes but so her often and worked closely with any of her help. She was 95 when she passed.
From what you've described, it does sound like she's well into this sad disease of dementia or alzheimers. Since going weekly to her Assisted Living Home, I was able to observe my Mother and the others in her home for well over a year.
Some have bitter and even violent attitudes ... and some only go there occasionally.
My Mother did call the police twice when under care at her home - and accused one of the caregivers of theft to the police. She also three times had episodes over night thinking and screaming that someone was in her house trying to kill her.
These things are very traumatic experiences .... to all .... especially an untrained caregiver and/or relative. You two need help in that area. Please tell your BF that the two of you need to get out at least once a week - to have some fun or privacy. More often is even better.
One of the most common problems with memory patients is distrust. I think it's one of the first signs. So finding and introducing a caregiver may be a problem ..... unless the one who works days (who grandma knows) can also work two evenings a week - for 3 or 4-hours an evening. Oh yes ..... and bad behavior at night (or after 5pm) is very typical of those memory illnesses.
I think if you had two nights a week away from grandma .... maybe one night to study at a library and one night to get out with your BF for some fun - but every week ..... you might get more mileage out of your patience. I think this would help the BF a great deal - even if he isn't showing signs of stress yet. He will - in time. Do something before that happens ... please!
Peace and wellness
Rob & Gizmo