I feel for your son. My high school (Montgomery Cty) was four miles from my home with no public transportation in the area to speak of at the time. Put a real cramp in my being able to participate in 'after school' activities since I didn't have a ride home.
You don't mention how old your son is. The distance is short enough to consider biking. Also, there are some buses, altho' they may not cover the area?
http://www.mtamaryland.com/maps/regional_system/index.cfmI would expect a private shuttle service to be quite expensive. Do you have a college/community college in the area? You might be able to hire a student w/ car to do it for much less, particularly if it is convenient (swinging by his school once a day).