My father has early onset Alzheimer’s. I believe he is in his late stages. How do I keep him entertained throughout the day?

It helps to think of your father as a 5 year old. Big picture books, photo albumns, helping you fold towels/laundry (even if it isn't done right). My dad was fascinated with young children and they liked him. Pull up a chair and let them color together. Old TV programs (westerns, old light sitcoms) were easy on my father; I think he liked having some sound on and was no longer interested in music. It was hard to concentrate by then. Don't expect too much.

I have the exact thought that JoAnn29 has. If they can fit a child with glasses they can fit an adult with cognitive difficulties.
How old is your dad? You say Early Onset but no age in your profile so just curious.
Are there any Adult Day Care programs in your area?
Is there a Senior Center that he could go to for some socialization and activities?
Is your dad a Veteran? If so the VA might be able to provide caregivers for a time (they will do a certain number of hours per month, I can not recall how many though) If he is a Veteran contact the local Veterans Assistance Commission and they can help determine what services he would qualify for. And there is a very good possibility that you can get paid for caring for him as well.

Look into adult daycare for dementia patients. They’re experts in having activities to keep people engaged

Since you are working and have a family to prioritize, I'm wondering if he has the financial resources to pay for some in-home aid, like a companion to take him for walks or do "tactile" exercises with him in the afternoon when he's Sundowning? Your ability to keep him occupied without also occupying your time will continue to diminish. If he enjoys watching tv or dvds that are light-hearted or comedies then it's ok to do so -- ALZ is "not good for him" more than anything. Or maybe consider adult day care for a few hours every day or every other day. Bless you for working so hard to care for him and the rest of your family...make sure to take of yourself (you need to put on your own oxygen mask first before you can help others).

First, if an eye doctor can fit an 8 month old with glasses they can fit your Dad. It has something to do with how the eye refracts light. But if Dad is in his last stages, he may not be able to read in as such he is not comprehending what he is reading.

Its hard to entertain someone who suffers from Dementia. If they didn't do it before the Dementia, they can't learn after. With my Mom, it was put the TV on a channel that showed all the old shows. I put her in Adult care 3x a week. If Dad has any money, you may want to try an Assisted living or Memory care. They have activities geared to people suffering from Dementia.