Create a new account (leftmost menu in the XMB), go over to where you'd usually go to log in to PSN manually, choose the... Err... I think it's 'create new account' icon, I'm sure you'll know which one you need when you see the menu options, it's the icon that isn't usually there. Choose the 'existing account' option then just enter your email address and password associated with that account, and you'll have your PSN account on his PS3. Unless you have Playstation Plus and the saves you want are in your cloud save area, you'll need to copy saves over manually with a USB stick. You can only copy saves; anything you've bought from PSN will have to be downloaded again and installed game data (i.e. the first time use install thing) will have to be installed again.