Is this a normal part of the aging process or should I be concerned?

After six years you know best! - but I sympathise with your generalised concern to check that all is as it should be. On the other hand, if he is happy then why upset his routine? So that he can live a longer, healthier, somehow more virtuous life? He's got to 85 and is content - he must be doing something right.

I agree that it would be a good idea to look for a specialist anyway, though; not least because when something changes you will need help, and then it's much better if you can call on someone who knows what is normal for your father, and whom your father already knows. Choose your geriatrician carefully - you want someone who is conscientious and thorough, but wary of tinkering.

And how are you? Are you managing all right? It is very pleasant for us to read of a contented, undemanding, unproblematic gentleman living peacefully in his daughter's home - but that doesn't mean we don't appreciate that all the same the responsibilities can take their toll.

Thank you for all your comments. He has been living with me for about 6 years. He is not depressed and he seems to be fine with the situation. He also eats cookies and candy. So he to likes sweets. I will look into a specialist in aging.

I would have him checked out by his doctor. Does he have diabetes? Heart failure? Depression? I use to think my dad was getting old and everything was normal, but it wasn't. I wish I had taken more time to encourage him to see a doctor sooner. I would make sure the doctor reviews his blood work. Maybe there is something more going on than just old age.

My mother is 81 and lives with me. What you mention as an average day's food is pretty close to what my mother eats daily. According to her doctor and most everyone I consult, it's normal. A decreased appetite is typical in the elderly, apparently.
In my mother's case, she would eat cookies, cake, or candy, two or three times a day every single day, and nothing else, if I didn't fix meals and put a small portion in front of her.
The love of sweets is also common, I am told.
I do agree that it's a good idea to get him checked out by his doctor. 

Sleeping more is not abnormal in old age. Sleeping all day and night seems beyond typical aging. Since he isn't expending any energy it is not surprising that he eats little.

I think a thorough physical exam is in order. Does he have a PCP he likes? If not, I suggest trying to find a geriatrician for him. (They specialize in old age the same way pediatricians specialize in childhood.)

How long has your father been living with you? I'm just wondering if it's been long enough for you to know your father's normal routine. If he goes along perfectly happily as he is, and has done for ages, then there's nothing to worry about; but if he seems not himself, or unhappy, or unwell, then it can't hurt to get him looked over by his doctor.

Nothing you've mentioned seems very unusual in itself - it's whether it's comfortable for him or not, really.