Well, one thing is for sure... the commute will be similar, if you live in Lorton or Chantilly. A lot depends on when he needs to be at work and what you consider a 'terrible commute'. The commute will be a lot shorter if he can get to work by 6, and leave before 3. I say 6 am and 3 pm, because this would allow him to use the HOV lanes before they are restricted, and also before it gets too busy. The commute will be around 30-40 minutes, if he can hit those times.
You mentioned that he has to be at work early... so it sounds like this might work. I think Lorton would be your best bet between the two options.
HOV Schedual:
http://www.virginiadot.org/travel/hov-novasched.asp