I think it's your own computer.So you can install a key-logger.It's a program that would keep a record of all the keys stroked on the computer ,including the chat sessions.Just be a little careful so your son doesn't get to know that his internet activities are under inspection .You know, it hurts a bit ,especially if he isn't doing anything that he shouldn't be doing.So,just be a little careful.
Oops I somehow missed the date on your post.