How to tell dad these relatives don’t want to see him?

It really is a shame that they can not spare 1 afternoon given his condition.
I would tell dad that they are busy and when their more able to stop by for lunch they will call.
Then say.. why don't you and I go out for lunch.
If he has friends that can join invite them as well. Keep the group small. And outdoors if possible if you have to be inside keep the group even smaller and require masks due to his probably poor immune system.
Bonus of keeping the group real small you can invite 1 friend this week and another the next time you go out.

Tell Dad he needs to call. You did it with no response. Remember, he needs you more than you need him. You may want to say when he mentions your not married that if you were, you wouldn't be able to care for him because your first responsibility would be to your spouse and children.

Leah, I wouldn't tell him that they don't want to see him.

I would tell him to get ahold of them, make arraignments and you will be happy to make sure he gets there.

Let them deal with the repeated calls and tell him themselves that they don't have time for a meal.

I am sorry that you are being caught between cultures. That must be soooooo challenging.

Local family members do not visit my parents. Some of it is due to Covid and some of it is because they have no desire. They call me occasionally and ask about my parents but they are living their lives and those lives do not include visiting elderly relatives they are not comfortable being around. You would like them they are not monsters. My parents have a very long history of an uncomfortable marriage and being in AL has given everyone an out. Add in my dad's untreated anxiety issues, my mom's endless list of ailments I can understand on some levels why they are not around. But I admit I expected more from them because these family members have been around for a long time. When I visit my parents these absent people are all they talk about--it is awkward. It is a lot of work to get the conversation to other topics. With the holidays coming soon this strikes a chord in me because don't think I can sit at the same Thanksgiving table anymore.
So....it is not your job to keep calling, texting, emailing these people. Let your dad contact them and let him deal with their responses.

You think they could be kind enough to accept the invitation with grace. I do not understand people sometimes. Tell your dad they are not worth his time. I do not think it is due to Covid. They can visit outside through the window or outdoors as some have done for my mother. One cousin is from Ohio.The other will visit from Georgia and talk through the window on the front porch next month.I bet if they were getting some money they would make time.