My elderly neighbor is 92 years old (WWII Vet) and suffers from diagnosed dementia. He can make himself a bowl of cereal and he eats lots of ice cream but is unable to cook or even use his microwave to make TV dinners. His personal care has really been diminishing lately and I wonder about him getting sick from bacteria. Also, his home has a lot of mice in it (evidence in the silverware drawer and cupboards). He has three sons, only one of which is within an hour's drive. The closest son brings food (bananas, milk, cereal, frozen dinners) once a week and seems to do no more than that. I have explained to the son what I have noticed but it hasn't seemed to help. I think he still sees his dad as someone from 10-20 years ago. Anyway, what else can I do except become a self-appointed care provider pro-bono?
So contact your local authorities, and let one thing lead to another until someone is taking a more practical interest in how your neighbour is coping. Have no qualms! You're not trying to get this lovely gentlemen evicted, and you're not accusing his sons of anything. You're just waving and asking for attention on his behalf.
I think I would document what you see for the agencies and for the son. Include the help you are giving him. Give it to all of them in writing. It may carry more weight.
Kudos to you for being a concerned neighbour. The man needs assistance, preferably from his family. Don't take it all on.