I checked with a friend whose daughter was in Prince Frederick (but in ACES not DCC). He said the honors program does require residency - either 1 or 2 years, depending on the program; but it sounds like you have a strong case to request an exception. I hope it works out for your son.
My understanding is that the “off campus” apartments are very good and located on the street across from the campus gates, so only technically “off campus”.
Good luck.