I'm going to tell my dad if you ever want to see your grandkids go to the doctor
Why doesn't your father want to see the doctor?
I think he's avoiding taking care of his illness because it's easier than facing the truth.
Your father may very much need the emotional support that your kids provide him with. To put that in jeopardy to get him to do things is wrong. You should instead have a conversation with your kids and your father present to reach an agreement about his health.
but what do I do since he is old enough?
Do not hold your kids hostage as a leveraging tool. Your kids will remember this. You can set up an appointment for him if he feels he has no time to do it or where to even start and that can be a first step.