I gave my son my old phone and he had his own number and Apple ID but all his info is still linked to my phone he can’t change contacts as it does the same to my phone how do I stop this ?
iPhone X
The same iCloud account is signed into his phone.
You did not properly prepare the phone for hime to use.
What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch
See -> 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
Remove that old iPhone from your list.
The only way that can happen is if the same iCloud or email account is signed in on both and has the contact sync turned on.
I have removed the device and am not singled in but the contacts are still linked nothing else just the contacts
See the link in KiltedTim response too.
How to unlink phones
