Changing phones isn't going to help. Set a good password and never let him touch the phone. He would have to have initial access to an unlocked phone to set up a way to remotely do anything to it. If he is around and you out the phone down, make sure you lock the screen before doing so.

For the current phone, i would perform a factory reset on it if you do not know what he has installed on it or done to it to give him remote access. Let it update if it needs to. If you need help setting up a phone, ask someone else for help, and clearly not him.