just a small note to add to what everyone has said, it depends on what I am painting as to whether I paint it separate or together. Like Scott I almost completely assemble the aircraft before I paint it, I just mask off areas where I don't want the colors overflowing the other. But for things like landing gear etc, I will usually paint those seperate then glue them in after the painted fuselage has dried. Some may do it different, but for me it is easier to handle a model for painting without the exterior delicate parts in place... (these feeble old hands would snap the gear off in a second!)
If it's cars you build I am sure someone into that more than I wil be able to answer your question, but I would think a lot of the car would be painted before assembly except maybe the chassis.. better leave that to a auto buff tho...
Good luck and welcome to modeling!
And look at it this way, even if your son didn't take to modeling, at least now you'll have a reason to ignore the video games
haha kidding... who knows, after you build a couple suddenly he might be interested...