You don't say how old your father is norma. I don't really know what to say, my father had alzheimers and it was awful. Some of the things you say ring true, apart from his face. Could it be that he has suffered a slight stroke? weight loss is associated with alzheimers but this is usually more noticable at the later stages.
MEn are terrible about getting their health sorted so you might have a bit of a battle. Forgetting things i think is pretty normal. My father i suppose did forget things but it was more that his perception and behaviour changed as opposed to him "forgetting" stuff.
What i would urge you to do is have a chat to your dads doctor, explain your concerns, then find a reason to get your dad to the doctor. I say this because there are treatments out there, that if given in the very early stages of dementia can delay the progress of the condition considerably.
I have to say that i wouldnt worry about the fly killer/weed killer thing. This sounds like the sort of thing i would do, and in fact i have done something similar - i had a mist spray for my seedlings, and one day i picked up the dettol spray by mistake. It could just be that he genuinely was on auto pilot, wasn't concentrating and picked up the wrong container. I know this is the thing that upset you the most and im not trying to trivialise it, but in context with the other things you describe, yes, its a concern but taken out of that context, something to have a laugh over.
Like other postors have said, it could be a number of things but early diagnosis is really important to all of these things. Saying that, at the end of the day, it is his life and only he can choose at this stage what he wants to do - making it much much harder for you.
I am sorry that you have these concerns and i hope and pray that your Dad continues to be well. The one thing is though, you sound lke a loving and caring daughter - he is lucky to have you.
The doctor will definately discuss with you your concerns, im not sure he will be able to tell you anything that he "knows" about your dads health without permission, but a general chat based on "im worried" "what if" "what do you recommend" etc. He will be able to point you in the direction of local organisations that could help.
With regards the weight loss, this could just be that he is slowing down and not as physical and therefore losing muscle mass. Of course, this does need investigating.
Best of luck