This is likely to be long - so apologies in advance!
My dad lives on his own (Mum died two years ago) and has a good relationship with his GP. He was tested for Parkinsons' disease earlier this year - thankfully the tests came back negative, but he has been given betablockers to manage his tremors which were limiting his activity quite severely (he couldn't hold a pen or a knife and had to drink through a straw). With the betablockers he manages OK on his own; while he still has a tremor it doesn't limit what he can do in the same way.
He also takes medication for high blood pressure and for asthma - when he does get asthma attacks they do tend to be quite severe.
Recently he has started having dizzy spells and had a couple of falls, which may be as a result of these spells. He doesn't sleep well at night and will suddenly fall asleep during the day without warning. He has also started mentioning hallucinations which are so clear that he will speak to them - he also gets moments of being very confused.
I suspect that the combination of medication might be impacting his health and I'd like to know how to help him manage - and to know if it is likely to be the medication or if I should be looking out for dementia 
. It's hard to watch when he remembers clearly something that didn't happen, falls asleep at 2 in the afternoon with a cup coffee spilling over his knees, or talks to an empty chair. What I'd really like to do is to talk to his GP, but since I'm not registered with the same one, is this something I can do as his primary carer? (He missed the last two appointments he had at the GP as he fell asleep before setting off, so they're aware of the problem) I could possibly talk to my GP about it, but without knowing the back story I'm not sure how much they can help.
Sorry for the lengthy post.