Chile is pretty awesome (if the time of year is right), if your son hasn't been to that area before. Missionaries in Chile tend to spend their time in the worst/poorest/least interesting parts of the country, or at least they did when I lived there. A week traveling around southern Chile (Torres del Paine, Lago Llanquihue, Saltos de Petrohué, Villarica, Pucón, etc) in the Chilean summer is a very different deal. Your son may not realize this?

Anyway, other cool options that are closer to Panama would be Guatemala, Colombia, and Ecuador. Guatemala has some beautiful areas, especially Tikal (spectacular Mayan ruins in the jungle), Antigua (colonial city), Lake Atitlán, and nearby volcanoes you can climb. Plus the major ruins of Copán are just over the border in Honduras, and there are lots of other archaeological sites to visit in Guatemala. Colombia has tons to see, include Cartagena, great beaches, mountain cities/towns near Bogota, colonial towns, etc. Ecuador has some beautiful highland areas near Quito and Cuenca, and a strong native culture.

If you want to go a little further, Peru has lots of awesome stuff to see and do, especially Machu Picchu, Cuzco, and the Valle Sagrado.