You have to read -> this Apple article: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch
What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support
Then you have to remove the iPhone / iPad from your account, see this Apple article:
Check your Apple ID device list to see where you're signed in
Check your Apple ID device list to see where you're signed in - Apple Support
Then your grandson can sign in with his own Apple ID.
Your Apple ID is the personal account you use with all Apple products and services. It includes the email address and password you use to sign in, as well as the contact, payment, and security details that you use with Apple.
You can’t merge multiple Apple ID accounts.
I had planned to set him up with a new iCloud email address
If your grandson has already an Apple ID, don't make a new one.
See -> Make sure that each family member has a unique Apple ID
Make sure that each family member has a unique Apple ID - Apple Support
- Avoid sharing an Apple ID