Observe the direct changes in behavior like consistent change in overall mood, shortness of temper (if it wasn't short before), lack of social interaction (if he was social before), memory loss (even forgetting mundane things can mean something if he used to have a mind like a steel trap), and then take that information to his doctor- without him. You and your mother should schedule an appointment to discuss the changes you're seeing. Most doctors will take your concerns and work with you for a resolution. Maybe he's just getting grumpy like anyone could at any age, maybe he's just a little depressed, or maybe it's something more. We always want to hope it's something simple, but the earlier you catch things, the easier it usually is.
Working in a pharmacy, I see all kinds of people with parents or grandparents who are going through all sorts of problems- problems that the children or grandchildren don't care enough about, and just get frustrated that dad or grand-dad can't remember their name anymore. You're someone who cares, and that's good.
The doctor may take your concerns and ask that you return with your father, in the interest of your father's care, so that you can all talk about it together rather than without him. That is also normal. Regardless, keep your chin up, and be proud of your father for raising a good kid who cares about her dad enough to care for him as long as he needs it.
