The rivers that come off the bay are his best bet. He can try off the city dock, or in the small boat harbor as well. I've caught silvers in Valdez through about mid September that are still chrome. Good luck!
There is only one river that is legally fishable, the Robe. It does have fish in September but is fly fishing only. Salmon fishing in all other rivers is not allowed elsewhere from Gold Creek east as it is a terminal harvest area.