You can still save Shaun, but Ethan will have to blindly guess the location out of 3 possible addresses. However, other characters may still be able to help --- I have nothing sig-worthy to say, but the post just feels wrong somehow without one.
So... if I did a playthrough where Ethan only does the first trial, I can still save Shaun? --- http://i49.tinypic.com/14w87bs.png http://card.mmos.com/psn/profile/an/o/AnonymousXIII/card.png Choking on a Lifesaver is the very definition of Irony.
Saving Shaun is more difficult if certain conditions aren't met. I've only played and beaten the game once, I did all the trials but one successfully, but I didn't kill a certain someone as required by the origami killer. I had the 3 other good characters available when I had Ethan Mars track down his kid. And they were actually a help to me in the final chaotic confrontation (The killer still tried to shoot me, even though I had technically passed his trials, so I assume he was too grief stricken to accept that a good father existed, luckily someone (several? It was too chaotic to remember) intervened at this time)
The phone/GPS showed 3 locations in the beginning of the scene, but I listened to the phone for a bit, and I heard seagulls, so I assumed it was near the river (being closer to the ocean?) and luckily chose the square [warehouse] on the right of the screen. I'm not sure if choosing wrongly locks you out of saving Shaun, maybe the others can still save them if Ethan doesn't make it in time.