They likely are not actively mixing now, but when you signed into his device with your ID and did not erase the device first you are asked if you want to keep data that's already on the device. If you said yes then that existing data was combined with yours in your iCloud. When you sign out that mixing no longer takes place, but what was mixed remains mixed. If you did not erase the device after you signed out then your things that were on the device when you signed out were mixed in with his.
Again, this mixing only took place when each of you signed in on a non-erased device. What you need to do now is go through your data such as photos and contacts and delete what got mixed in. You may also both wish to change passwords wherever you used them since potentially some of yours still reside on his phone, and you may have some of his.