All your boyfriend needs to do is delete the existing account from his phone, migrate the iCloud data to a new account with his own ID, then delete your data from his account.
Have him start by saving any photo stream photos that he wants to keep to his camera roll by opening the photo stream album in the thumbnail view, tapping Edit, then tap all the photos you want to save, tap Share and tap Save to Camera Roll.
Once this is done, he should go to Settings>iCloud, scroll to the bottom and tap Delete Account. (This will only delete the account from this phone, not from iCloud. The devices that will be keeping the account will not be effected by this.) When prompted about what to do with the iCloud data, he should choose Keep On My iPhone. Next, he needs to seup up a new iCloud account using his own Apple ID (if he doesn't have one, he can tap Get a Free Apple ID at the bottom). Then he should turn iCloud data syncing back to on, and when prompted about merging with iCloud, choose Merge. This will upload the data to his new account.
You will both then need to go to icloud.com on your computers and sign your iCloud accounts and manually delete each other's data from your account.
