So here is what happened.

Your dad’s dad has a great head of hair because he didn’t inherit the baldness gene. So there is no chance that your dad got the baldness gene from his dad. But now you will have to look at your dad’s mom’s dad. Your dad got only one baldness gene and that’s from his mom’s dad. Because he got only one balding gene, he started balding late aka when you were born. If he had 2 balding genes, he would have been bald any time between age 18 to 30.

Now your grandfather is totally irrelevant to the topic about your hair. You will have to look at your father and your mom’s father. Because your father has one balding gene that he got from his maternal grandfather, it has a 66% chance of getting transmitted to you. There is a decent chance you might not get it as well but we’re considering the worst case scenario.