How can I help my father get treatment for memory loss? He refuses testing, medication, and his memory continues to decline.

As Alva said, “You well know what you are likely looking at”. One approach to your father would be to ‘diagnose’ it yourself, and simply tell him the diagnosis. Tell him if he doesn’t want to look for medical help, you will move forward making decisions on the basis that your diagnosis is correct.

This is a good time to look at what you and he need to get in place immediately – a POA, information about F’s financial options for care, changes to where he lives at present. Being firm and clear may be more helpful than continuing to put pressure on him to ‘find out the worst’.

https://www.agingcare.com/topics/13/memory-loss

You have posted this under dementia.
You well know what you are likely looking at, though it would be some comfort to have diagnosis in so far as medical science is able, and any idea of prognosis.
As I am certain you are aware, we as yet have no proven treatments, and treatment goes according to symptoms. The idea now is to keep your elder as safe and provide for as you are able. I wish you the very best, and am so sorry you are going through this. Know here that you certainly are not alone.