With that budget range, you may find a room share somewhere. Not a lot of options, but generally you'd want him to stay somewhere near public transportation. Look up MUNI, BART lines, and wherever you're deciding on, make sure there are a few public transport options within walking distance.
Sounds like your son is working near where I am, not a lot of residential within the immediate vicinity. Mostly are highrise condos/apartments, and they're way out of your price range unless you want your son to get stabbed everyday going to, and from wherever he's going. Look a bit further out west, definitely
NO Bay View, Hunters Point, Tenderloins, or anywhere near those parts...
Public transportation is the method of choice of getting from one place to anther here, and you can get anywhere in the city with one transfer. Just to give you an idea, a one way commute on public transportation from the western edge of SF to that area is about 1 hour, so plan accordingly.
Try this
http://sfbay.craigslist.org/sfc/roo/
Beware of SROs, and hostels.