I have his password for everything

I had asked my bf one night to let me use his school login to log onto the computers at his school to do some work he gave me it and I thought his password was really funny and he’s like yea that’s my password for everything.....I’m in this moment where I don’t trust him, because he had been talking and flirting with other ladies. The other night I had asked for his Facebook password and he said yea I’ll give it to you but tomorrow..like what so he could have time to delete shit? I want to try and log on with the password he gave me. I know it’s invasion of privacy but last night I was up for hours shivering because I kept feeling like something is up. Should I log on or just leave it.