Does adapter need to be unplugged and back in before I can connect to it?

Did your machine go to sleep? As in did it enter sleep mode (screen goes off and usually the power LEDs on a laptop/machine indicate a crescent moon icon to signify sleep)

Just did that. Thought it would work but it didn't.

When connected with USB and the wireless adapter is on and connected, when you unplug the controller does it turn off or does it connect to the wireless adapter?

If it turns off after unplugging it from USB you will need a new controller.

Are you using wireless of wired connection? If you're using wireless with the the old version of the xbox 360(the one that doesn't come with a built in wireless adapter) try unplugging the adapter, turning off the xbox, plugging the adapter back in, then
turning on the xbox. If that does not work try connecting your sons xbox directly to the modem using an ethernet cable and see if you can connect.

Hmm, it may be something specific to the wireless adapter. Let's try this: if you unplug the adapter, will the controllers connect? If they will, then please try plugging that adapter back in to another USB port and see if that helps at all.

Try unplugging the adapter and plugging it back in. You could also try going to your wireless settings on your xbox, select the network your connected to, forget it, and then re-connect again. That has helped me before. If neither of those work try to
connecting it to wired. I know you said it's out of the question but if you could try it even once, we could see if it's the adapter with the problem, your internet, or perhaps the xbox itself.