Welcome to the server! I wouldn't worry about AA's at this point. Wait and see if you get to that point first. And if you do, all of the classes have something nice to look forward to.
Since you're son is relatively young and you're looking for something relatively simple, I'd avoid classes like Enchanter, Necro, and Bard as first classes. Those classes are very powerful and versatile, but can be pretty intimidating and are also pretty fragile. On the other hand, a relatively passive class might also be a poor choice as he'd just get bored (or not, you know your son best) - so classes like Cleric and Wizard are out. I think a hybrid is a great choice for someone so young who is also new to the game, as the melee side of the class means he is guaranteed to have something to do every fight while the caster side means he still has spells to keep things interesting. So classes like Paladin, Beastlord, Ranger, or Shadowknight are all great choices. The Beastlord in particular is especially nice since he can contribute a lot even just by sending in the pet while he gets more comfortable with the rest of the class. If none of those classes sound good, then I think other solid options are Mage, Monk, and (to a lesser extent) Rogue. In general, I recommend just asking him what he thinks sounds the most fun and going with that. If he gets really excited at the idea of being a kung fu master, have him go Monk. If he wants to be a holy knight, go Paladin. He wants to be sneaky? Rogue. At the end of the day, if he's excited at the story of his character, it'll help him enjoy the game even more.
Once your son has chosen his class, you can select yours to match his. In general, Shaman will synergize the best with any of the above choices. Specifically, possible combinations that will definitely be strong are:
ROG/SHM (you tank as shaman with slow, heals, and dots - he backstabs)
But really, I just hope you have a great time playing here with your son. At the end of the day, if you guys have a blast playing a warrior and a wizard, then that's all that really counts
One last thing, don't be intimidated about picking a melee class on this server. There are really solid melee weapons that are cheap or given away on a daily basis - so no need to use rusty weapons for the first 20 levels!