Wow what an exciting letter.
And I figured out where he is.
Ilopango is a town a few miles east of the nation's capital, San Salvador. This town is located next to Lake Ilopango, the country's largest lake.
Lake Ilopango is a Caldera, which is the center of a volcano that has erupted, and then filled up with water. The last eruption at Ilopango occurred in 1879-80.
Attached see the picture taken on 11 Nov 2009 from the International Space Station.
The city of Ilopango borders the lake to the west (image left), while green, vegetated hills ring the rest of the shoreline. White, patchy cumulus clouds are also visible in the image (center and upper left). There is an airport located in Ilopango that is used by charter flights and the Salvadoran military (image lower left).