For FPS games your ping will be far too high.
However the bigger issue is the traffic shaping they perform when you try to play. Having played with a mobile broadband connection (Three - a UK company) for 4 weeks, I've discovered that regardless of how little bandwidth your game uses - say 3k/second, the network will cut you off if you exceed even 2k/second after about 1 minute.
Presumbly they have some form of neural network or learning software in place that blocks your UDP packets after a set period. It may not be the same on HughesNet but I'd be wary.
It's crazy that you can download files, browse 500k per page websites like Flickr but they won't let you stream far less bandwidth for games.
The solution I use is to host an adhoc wifi network on Windows 7. Then once the game is blocked, I kill all the connections to that game using Netlimiter. The number of disconnects I get generally drops after peek hours, but they're very aggressive with the "learning" and blocking of the gaming traffic during peak hours.
It's ugly and extremely cumbersome but does work slightly better than a reconnect via the modem.
You might also want to get an antenna for the 3G USB modem, either a clip one or an outdoor one.
I would suggest buying your son a prepaid card. You do have to remember when it runs out and buy another, but they tend to be $10-15 cheaper than buying the same amount of time through Microsoft directly. It's also much easier to stop being a subscriber, since you just don't add more time to the account - no need to figure out how to stop the automatic payments.
Best Answer
I've been trying to do this for weeks and I set myself a rule that I wouldn't call CS nor cancel my DD. As a UK user the "Automatic Renewal:" status isn't a link:
It's ridiculously simple to turn it on, but turning it off the same way is deliberately impossible.
Edit: http://forums.xbox.com/9/31086471/ShowPost.aspx#31086471 (FAQ & Guide - How to turn Automatic Renewal OFF Please read) contains the gem "Note: The Auto Renewal ON/OFF feature was removed from all markets except USA, Germany, Taiwan and Japan on the 18th of February 2010." Wonder why..?