1. Has your son asked you to change ISPs?
Most American families use trusted and responsible Internet Service Providers, such as AOL. These providers have a strict "No Hacking" policy, and take careful measures to ensure that your internet experience is enjoyable, educational and above all legal. If your child is becoming a hacker, one of his first steps will be to request a change to a more hacker friendly provider.
I would advise all parents to refuse this request. One of the reasons your son is interested in switching providers is to get away from AOL's child safety filter. This filter is vital to any parent who wants his son to enjoy the internet without the endangering him through exposure to "adult" content. It is best to stick with the protection AOL provides, rather than using a home-based solution. If your son is becoming a hacker, he will be able to circumvent any home-based measures with surprising ease, using information gleaned from various hacker sites.
2. Are you finding programs on your computer that you don't remember installing?
Your son will probably try to install some hacker software. He may attempt to conceal the presence of the software in some way, but you can usually find any new programs by reading through the programs listed under "Install/Remove Programs" in your control panel. Popular hacker software includes "Comet Cursor", "Bonzi Buddy" and "Flash".
The best option is to confront your son with the evidence, and force him to remove the offending programs. He will probably try to install the software again, but you will be able to tell that this is happening, if your machine offers to "download" one of the hacker applications. If this happens, it is time to give your son a stern talking to, and possibly consider punishing him with a grounding.