Depends on whether your son wants to game the bigger battles of the war, such as Brandywine or Monmouth, with formed units, or if he wants to fight skirmish level games such as those depicted in "The Patriot." One of my friends intends to fight mostly skirmish level games, so he bases his figures on 25mm square steel bases. If he wants to move the figures in groups or represent a larger scale battle, he can just set the ferrous metal based figures on magnetic strips 50mm wide and however long he wants them to be. We recently used magnetic pieces the size of business cards for this purpose. That's what I would recommend.
For rules, some people use Brom's Standard Rules for many periods including AWI. It is similar to The Sword and the Flame colonial rules but for larger scale battles. I think it might be a free download at the Jackson Mississippi Wargamers home page. If it's skirmish scale gaming he wants to do, then he could adapt The Sword and the Flame for muskets or could adapt Sharpe Practice, the rules meant to recreate the adventures of Bernard Cornwell's Napoleonic Sharpe series. The weapons are about the same for AWI.
The north is where many of the famous and decisive actions occured. However, your son might want to consider setting his action in South Carolina. There was a huge number of small actions and battles there, many of them small clashes over individual farms or plantations. If I were doing AWI, and I'm thinking about it, then I'd seriously consider setting my campaigns in South Carolina.