By "reader," do you mean a collection of primary sources, or a secondary text? Primary sources are hard to come by with pre-1850 African history because there were few written languages south of the Sahara. Now, the text I would recommend for this is often taught in introductory university classes, so I'm uncertain how suitable it would be for high school teaching. You can take a look and see if it fits your needs, but it's The South Africa Reader by Clifton Crais and Thomas McClendon.
Adam Hochschild's King Leopold's Ghosts is a history of the colonial Congo, which you may find significant to your son's studies, although this book is quite brutal. Cheers!