Father in denial, any suggestions?

My father is a very big, proud, strong man. He was a professional athlete and likes to project a superhero persona to us all. Unfortunately, over the past year we have noticed his memory slipping: putting things away in strange places, losing things, forgetting something that happened 5 minutes ago but telling stories from his past, foggy look to his eyes, walking strangely (may just be his bad knees) We are quite certain he is diabetic- he tested his blood sugar last fall and it was very high, so he went on a crash diet and lost weight and said he cured it himself. When he did lose some weight and his sugar went down, his memory did seem a bit better. Now he is eating all the time and we are sure his sugar must be very high again. Here is the dilemma: because he is super man, her refuses to go to the dr to get things checked out. He has a dr and is on blood pressure meds; my mom told me today that he probably hasn’t been for an appointment in a couple of years. He always makes an excuse to cancel the appointment and the dr just refills his prescriptions. Complicating things are two issues: his mother had severe dementia and his sister who is a year younger is in a care home with dementia as well; getting dementia is his biggest fear. Second issue is the fact that he has one or two damaged knees from his football days that he denied there was anything wrong with and also will not have them looked at- they have gotten progressively worse to the point that he has to hold onto things to get around and has to crawl up the basement stairs. Even with this obvious decline he insists they are fine and that he goes running 2 miles a day?!?! When we have approached him to go see a dr we are often sworn at, verbally abused, told that he can still do everything better than our “fat asses”, etc. He then continues the verbal abuse on my mom when we leave. She can not be his caretaker as she is in kidney failure and has COPD herself. The verbal abuse is not helping her either. How do we help him? How can we talk about this with him when he firmly believes he is invincible and then lashes out at my mom for weeks afterward? My siblings and I are totally at a loss for how to deal with this situation. I told my mom that I would phone his dr to tell him what is going on and insist on bloodwork before refilling any prescriptions. Mom and I think he will not go to have blood work because of how poorly he walks and his high sense of pride, and will probably just quit taking his blood pressure meds too. My Mom does have a home care nurse that comes by every 2 weeks to assist with needles she has to take. Maybe the dr could tell my dad he knows the nurse goes there and she could take his blood while she is there because they don’t want extra people at the lab during COVID? Grasping for ideas. This is a very difficult spot to be in and we are looking for any advice or direction anyone can offer. Thank you.