I go see my dad once a week most times. 3 or 4 weeks ago he had some kind of flu or cold and the staff at the nursing home said there is no sense me going to see him because he was mostly in bed sleeping. So I skipped one week of seeing him. When I went to see him the following week, he seemed lethargic and very weak. So I thought maybe he was still feeling the affects of the flu or cold. I didn't stay long. Then I went again a week later and he was even more lethargic. He kept staring off into a corner and speech seemed very difficult for him. Is this a normal progression of the dementia or could this be caused by medications?
I have found that my mother takes a step down with her dementia from time to time, meaning she's fine one day and then BAM, she's declined to a new level the next day where she stays for months before she suffers another decline. This may be just such a decline for your dear dad. Get the nursing home's perspective on things as they see him daily and have a better feel for his general health.
I'm taking a palliative care approach these days with my mother, meaning I'm not looking for a 'cure' for each of her ailments or rushing her to the ER every time she has a complaint. Yesterday, she was going on and on about a large bruise on her butt and insisting it's cause to go the ER. It's not, and we're not going that route, at least not for a black & blue!
You might want to speak to the staff at the nursing home about your dad's decline and get their feedback on what's happening. And schedule a doctor's visit for him if you feel it would be helpful.
Best of luck!