Yes, as long as the rogue is still undetected (aka has not fired a shot yet and is hidden), he can sneak attack (once), because all opponents not aware of him will have their dex bonus denied.
As soon as the first shot is fired, however, opponents will (most probably) become aware of him and therefore regain their dex bonus. Since they are not flat-footed, they will regain it immediately and so only the first attack (if multiple attacks are available) would gain the sneak attack benefit.
There will not be another surprise round or something, he'll roll initiative like anybody else and just remain hidden and probably delaying until he wants to come out of his hiding place, fire his shots and then enter combat normally (or move away, hide again, come back and repeat, which will take at least 2 rounds to prepare or a -20 penalty to hide).
Bye
Thanee