The admin password and the WPA keys should not be the same. You only need to give him the security keys not the admin password.
The bandwidth limit is a tough one. You can limit where he goes with a router that has a firewall (ie you could block high volumn sites) but to track usage and block after a limit is not a common feature. I would expect to see this feature soon with the number of people asking this question and with the growing popularity of wrieless broadband (ie cell phone based internet) which has expensive caps on them. If your router has the option of load dd-wrt software you may want to try that. This is community written firmware and has more features. The largest issue with doing this is you really need advanced device that understands calenders and when caps reset and also have the ability to save usage to some media so that it can tolerate a power outage. I know of one kid that would unplug the router to reset limitations.
There is software that will run on a PC that track usage and places limits. It just depends if you can trust the kid to not just disable the software.
`I want to stop my son getting access to Wireless Network password but allow him to have Internet access. He is doing IT at TAFE. I have just set up a new password on new ADSL2 router. He took over as administrator of my old network, I want to prevent him from doing this again.
How do I allow him access to Internet but prevent him from hacking network WAP password?
Can I limit how much he downloads to 100GB on his desktop and laptop, I have 200GB limit each month that he keeps using up and then others in house have to put up with slow internet speeds for the rest of the month? Everyone has their own computer in house.