Hi,
Changes in appearance like those that you mentioned, can be a sign of dementia. However, if this is the only indication of a behavioral change, there could be another reason for it. The Alzheimer's family history is certainly something that bears a closer look in this situation. Does your father have any memory issues that have recently become apparent? Is he still in touch with where he is, the date (month/year) and who our current president is? Does he repeat questions that have been answered or tell stories repeatedly?
If your father has no other symptoms and appears to be functional, you may want to observe him for a while to see if anything else is going on. Check the refrigerator for outdated dairy products or old cooked food that should be thrown away. Neglect of food safety and the laundry could be some red flags that things are no longer being well managed in his life. If that isn't typical for him, something is probably wrong. It may not be anything really serious, though.
Hopefully, your father's repeated use of the same clothing is just a response to economic pressures and he's possibly trying to get a little more mileage out of them before washing them. If so, it's just a lifestyle choice, even if not one that you or I would make! You might tactfully approach this situation with him and ask him if his washing machine is broken or if he is out of detergent. There might be a good explanation that would put you at ease and might even provide an opportunity to help him with a minor problem. Maybe the water bill or electric bill got his attention and he's determined to reduce costs. I hope that it's something simple like this.
If it appears to be something more, convince him to see his regular doctor. The doctor will check him out and order some tests to see what might be causing these things. When all of the common things have been ruled out, his doctor may refer him to a neurologist for cognitive testing and a CT scan. His family history will be part of that evaluation. If dementia is the probable diagnosis, there are some good treatments available in the early stages. Living alone will not be a safe environment for him anymore, either. It will be the beginning of a long and challenging journey.
I wish you and your father well. Please let us know how things turn out. Best Wishes, -- Joe