I would poke around the WRT54G admin webpage, there may be something there for you. As for a utility to run from a desktop, doubtful unless one came on the CD and doing something custom would require it to work via the webpage since Linksys isn't nice enough to give people shell access. If you really want to, that's probably one of the models that you can flash with a custom firmware, then you would have an almost fullblown Linux install complete with an ssh server and from there it would be simple to write a script to ssh in and take down the wireless interface.
Also looking for SIMPLE software that constantly monitors all connections...anyone?
Do you have any idea what that would entail? Just opening a single webpage initiates between 1 and 10 connections on average, more if there's a lot of data on different hosts or you've messed with your TCP/IP settings. If you want to monitor what he's browsing you're better off getting software specifically for that or installing a proxy on another machine (one that he doesn't have access to) and setting your router to only allow HTTP requests from the proxy so that he can't bypass it.
If you want to monitor more than that you'll probably not find something you would consider simple and you would end up reading through a lot of logs, so much that I doubt you would catch anything anyway.
Look at it like this, my firewall (a custom Linux box) has been up for 32 days and in that time it has NAT'd 97,926,758 packets totaling around 54G of traffic and My local workstation which has only been up 4 days has initiated 26,345 outbound connections. Yes, that's probably extreme and a lot higher than your son will be doing, but if he accumulates only a quater or an eighth of that number you'll probably be overwhelmed.