Is it okay for my grandson to call his grandpa by his name when he is five years old?

This title is generally considered bad.

In traditional Chinese culture, calling an elder by his or her name is not considered polite or respectful. Especially in families, grandchildren should use appropriate titles to address grandpa. Such behavior can express the grandson's respect and respect for his grandfather, and also help maintain the family relationship.

Of course, specific situations may vary depending on family culture and personal habits, but generally speaking, respect for elders is an important value.