Do you want the good news or the bad news first? The good news is that AT&T Wireless does indeed have a system called "Smart Limits" that lets you control quite a bit about the usage of a given cell phone, whether it's incoming calls, hours of available service, or specific download limits on songs / ringtones, etc.

The bad news is that it isn't free. It's $4.95/month at the time of this writing and while I believe it should be a free additional capability for anyone who is paying for more than a single line (e.g., parents), I'm not in charge of AT&T's financial picture and have no ability to change their pricing strategy. Frustrating, but since my cellphone bill is already ridiculously high, I just sigh and get on with my life. :-)

To enable smart limits on your son's wireless cellphone, you'll need to start by logging in to your account at AT&T Wireless. Once you do, you should see an account overview similar to this:

Near the bottom look for the "Manage my account" choices:

The option you want if you don't yet have Smart Limits enabled is "Manage features", which will show all the phones in your account. Click on your son's phone, then scroll down to find "Smart Limits" and enable it.

Now to configure or modify the Smart Limits settings, go back to the top and under the "Wireless" tab you'll find "Smart Limits" under the For Your Security section:

Now you can see all the useful options and capabilities enabled with Smart Limits:

For the late night texting problem, you'll want to select "Time of Day Restrictions". Click on that and you'll be shown the current settings, if any:

It's a bit hard to interpret, but I block all incoming and outgoing communication other than calls to known numbers from 11pm to 6am on this, my daughter's cell phone.

To change it, I'd simply click on "Edit" and can modify or tweak things as appropriate:

That's all there is to setting time limits on your AT&T wireless cell phone service. I hope it's helpful. Still, I'd talk with your son if I were you too, so they don't think something's broken... Good luck!