Why is Washington DC included in the Southeast Region of the US?

There are no fixed regions of the U.S. The postal service has their regions. Some try to distribute their regions equally (e.g. population). It depends on what your son is studying. I know Maryland is included as a southeastern state on some maps. Washington D.C. is usually considered on the boundary between the southern states and the mid-atlantic states.

Regional maps

U.S. Census Regions

http://www.enchantedlearning.com/usa/statesbw/regi...

Maryland as a mid-atlantic state

http://library.thinkquest.org/4552/

Virginia & Maryland as mid-atlantic states

http://www.mapsofworld.com/usa/usa-maps/united-sta...

News to me Regions

http://www.manifold.net/cases/cd_map/cd_map.html

With the exception of the federal government and the more northeastern culture it brings to DC, the city is culturally otherwise a southeastern city. It also sits roughly in the geographic and cultural gray zone between what some may consider northeast and southeast. Maryland, DC, and northern Virginia all mix traits of the northeast and the southeast. Although it really depends who you're asking as to what you group DC with, different gov't agencies each have their own ways of splitting up the country. For example the Census Bureau lumps it in with the South Atlantic region.

Maryland was in the Union Army in the War Between the States; Virginia was in the Confederate Army. That may be partly the reason.