When trying to reset, I enter correct apple ID PW yet it says incorrect

Here is what's going on. My step dad gave me his old used iPhone 5s that I'm using strictly for music and not to put service on. When I go to do a full reset it asks for his apple ID password. When I give the password, the box shakes like I entered in the wrong password even though it's the correct password for his account. I have already logged into his iCloud to take the phone off of his account but it still won't let me reset without his "password". I tried connecting to iTunes and resetting that way and it tells me I have to turn off my "find my iPhone" setting but once again I have to put in the password that it says is incorrect. I have also tried holding down the home and sleep/wake buttons and it goes to the white apple screen but doesn't reset. How can I resolve this without going into an apple store?