In no particular order, I'll bullet point some things to (help) answer your questions.
* First, as you surmise, there is NO way to fly co-op in a Single Player environment. You must, as you realize, fly in Multiplayer to do that.
* You can get into MP in a few ways.
-- You can start your own using your own game (ie. the Create Server option)
-- You can create one using the standalone DServer.exe file in your game's installation area: --> YourDrive:\YourRoFArea\bin_game\release ...dserver.exe. There's some setup knowledge needed for that, but I think there are still some guides around here, and in the BoS forums -- since it's 85% the same software, and 95% the same procedures to help you with that.
-- You can log in and fly in any of the current MP servers that are hosted by us (at New Wings), or a few other server operators out there.
In any of those MP environments, you'd be able to fly co-op with friends either as separate pilots (ie. you both fly separate aircraft), or as pilot & crew in a bomber or 2-seater ... IF:
The Mission(s) being hosted IN the MP environment you've chosen have enough planes for you all (usually easy), and / or bombers or 2-seaters you can also pick (will depend on the mission itself).
Regarding AI, that too *must* be built into whatever mission the MP Server is running. In many cases, since MP servers are / were intended for PvP (Player vs Player), many of those missions have minimal, or no AI, and they're there, mostly for a little "flavor". For instance, most of our New Wings - Wargrounds missions do have some AI aircraft you can find and kill, but it's pretty minimal.
BTW, if you don't know this already, you can "copy / borrow / steal" any mission that is hosted on any MP server you enter. Those missions get downloaded to your computer as part of joining the server, and you can grab those files from your PC, load them into the Mission Editor, and tweak, adjust, and save them any way you see fit -- so that's one way to *get* some missions for a server of your own.
As Panthercules also noted, you can also use PWCG to create missions, save them, and then with a few editting tweaks, make them suitable for Co-op play by hosting them on your own server. Alas, like Panthercules, it is so long since I've done that, I also forget exactly what, or how, they need to be changed in the Editor to work that way, but it definitely can be done.
So, to sum up, you *must* have a MP server, and it *must* host a mission that has the various planes, objectives and AI that you'd like to deal with to achieve what you're looking for -- it's doable, and has been done in the past, but you're probably going to have to dive into it yourself to do it as most folks have moved on, and the ones who remain are largely SP folks (PWCG or RoF Career), or just flying on the existing MP servers.
Which reminds me... In our New Wings - Basic Training server, you can definitely fly the various co-op planes in there (it's a Non-PvP environment) against various AI and targets, but it's not really mission or objective oriented like a Wargrounds, or PWCG mission would be... Still, it's a great place to fly with your son, and help him learn how to be a good gunner and crewdog!
Hope that helps!