|He couldn't access the ISPs logs but, depending upon how clever he is and how secure your setup is, he could certainly access your Mac remotely and access any data on it. Private browsing will help with this, but can be a bit inconvenient. In essence, you should never trust anything that you do on a computer to be confidential unless you are very well versed in computer science.|
I still think the big problem here is that you are living with someone that you don't trust and that you are doing things you don't want him to know about. That's not a problem that anyone here can help you with.