On the last page, you asked about keeping units in cities that were not on the border. It is standard practice to reduce your forces to a skeleton crew (at least under the amount that you'd have to pay for). But the AI knows which untis you have in any given city. If you leave a city ungarrisoned, the AI knows this and the more aggressive Tribes will try to send units in to take it. You can temp them with this ploy to start a war. Conversely, if you keep decent units in all of your cities, this temptation does not surface. Just a quick suggestion on how to keep the AI civilised and friendly. Another way is to offer to trade World Maps. Ask what they will give you for your WM (or Territory Map). Communication with the AI keeps them off your back.
About your game running slow, there are articles here about how to speed up your game. Hints like going to your Preferences screen and unclicking the options for animation, things like that.
Glad you're playing with your son.
BTW, I enjoy playing solo just as much as I enjoy playing with humans.