separating icloud accounts

I have an iPhone 7 and my husband also has an iPhone 7 as well as an iPad. He does not use email, so all three devices were set up using my email. He does actually have an email address that has only been used to set up a Facebook account. He needs more icloud storage on both devices. However, I realized when he got these new devices that everything from my account was backing up to his devices and I believe if we just click to add storage that we will continue to back up everything on both accounts.

What is the best way for me to separate the accounts? Can I simply edit out my email (or icloud) address on his devices and put in his email? Will this preserve his content and separate future content?