My mother too is in her 80s and a diabetic on insulin. She had fallen over a few times and each time it was due to either high blood pressure because her sugar levels were not under control and on other occasions she had an infection (usually a urinary one).
Older people with diabetes can get urinary infections so it is worth checking it out.
The things to watch for are confusion and observing the colour and smell of the urine. If it is very strong smelling, very dark (can indicate dehydration or blood), or is cloudy, this usually indicates a urinary infection.
Old people are not always able to tell you what the problem is, so by observing the person, you can learn quite a bit and inform the doctor.
Another problem that older people and those with diabetes can have is low Vitamin B12 levels, a blood test would confirm whether this is the case and the doctor would prescribe a course of injections of this Vitamin. Sometimes this vitamin is not well absorbed in the gut by some people.
If there is any concern regarding the health of an older person, it is always wise to get them checked out by the doctor. It they are unfit and unwell to travel to the doctor's surgery, the doctor can do a home visit.
My step Mother is 83 and she has started the same thing. She has a bulging disk in her back and it has pinched a nerve. She has slushiness also and a diabetic. I went and got her a wheel chair and walker because when older people fall especially being a diabetic it's so hard to recover
Get him checked out by the doctor as soon as possible.
If the old chap is a diabetic, he may not be controlling his sugar levels due to forgetting to take his medications.
Another reason for older people to get forgetful and falling could be due to blood pressure or having a urinary infection. The doctor will do a simple urine specimen check and may prescribe an antibiotic if there is any indication of an infection, and/or send the urine sample to the lab to check for the specific bacteria so that the appropriate antibiotic can be prescribed.
For the vision problem, because he is a diabetic, he should have his eyes tested with regard to making sure that there is no damage to the back of the eyes caused by high sugar levels. Not all opticians do this type of test, so you would have to ask as say what the test is for. An eye sight test is not the same as having the eyes checked for any damage due to diabetes.
Forgetfulness in an old person may be age related and it could be the onset of dementia, but it is best to get him checked out and especially for any urinary problems.
Let me know how you get on.