Looking at the WABA site is a good suggestion. Also, look at these sites:
Anacostia Tributary Trail System map (Prince George's County):
http://www.pgparks.com/places/maps/anacostia-2002.pdf
Montgomery County Park Trail Maps:
http://www.mc-mncppc.org/trails/trail_maps.shtm
As for a route from the Mt. Rainier area to your husband's job (since he works pretty far out Colesville Rd./Columbia Pike), I might suggest that he take the Northeast Branch Trail towards College Park, then pick up the Paint Branch Trail near the College Park Airport/Lake Artemesia/College Park Metro station. Take that to Cherry Hill Road, then head west on Cherry Hill (bike lane for some distance) to Colesville Road/Columbia Pike (see the Anacostia Tributary/PG Co. map for more info...).
He could also take the Northwest Branch trail towards Montgomery County (see both the PG and MoCo maps for more info...). This is more direct, but it seems like the paved portion ends just inside the Beltway, and a good distance short of Colesville and where your husband needs to go (unfortunately, it seems that only major roads cross the Beltway nearby, and that the connectivity of the smaller roads isn't too good between different neighborhoods...). Maybe others would have suggestions as decent/not too busy local roads that he could take from there...