Can you tell us
what computer OS, (Operating System), you are all using?
I don't know anything about XBox or Wii, or Mac OS, but in Windows you can restrict the time anyone is on the internet. Depending which Windows, Vista and Win 7 have built in Parental Controls in the Control Panel. XP didn't but it is easy enough to set internet time limits with a Command Line instruction as described in the link below;
But now comes the difficult bits. Any youngster who knows computers or who knows how to find out more, can override anything parents can set, so the trick is to make it as difficult as possible for them.
In Windows that means letting him have just a Limited User account with no Admin privileges. But you, or whoever admins the system, must set all other Administrator accounts with different STRONG passwords. That means a password nothing less than 10 characters, using lower case and upper case letters, numbers and regular symbols like the ones above the number keys. No common names, phrases, pin numbers, dates of birth, etc allowed.
And don't forget the hidden System Administrator account. If you forget that, your son may not.
Don't forget the passwords!
The difficulty with suggesting free parental control software is that they are all different, and users will have their own preferences, so I could only suggest looking at reviews, like this one;
As to the other devices on the internet, eg the XBox and Wii, and also the computer itself, there are things you can do in the router to block sites or may be even block access to the internet, and I see from my own Netgear router's access pages that I could even do that remotely from the internet.
Have you ever accessed your router's setup pages? You do that from within a browser window and the manual that came with the router will tell you how. If you haven't then it is likely the login screen is still username=admin and password=password. If so you would need to change the password immediately to some strong password, (and again, don't forget it), otherwise anyone can access it.
Finally, and sadly not to put your mind at rest, a determined person, (we used to call them 'script kiddies' because they were often just teenagers), can break any of the safeguards put in place and gain access to any account.