He was sitting all the way in the back first of all. Not right next to the cockpit.

So think about it.

His ears start popping and he starts feeling pressure in his head/probably getting a little dizzy and then suddenly the oxygen masks drop.

Instinctively, he puts it on and waits like everyone else. Anxiety kicks in and his heart rate sky rockets. He sits there and just....breathes. Because what else would you do at that moment?

So twelve or so minutes goes by and he sees everyone start to pass out. Now he realizes his will run out soon too so he frantically starts looking for more oxygen masks. More time passes.

But during this time he probably has a bit of hypoxia himself and might not be thinking completely clearly, which would be to make a bee line to the cockpit and try to contact them, but maybe thats not protocol so he doesnt.

So hes going from mask to mask that are hanging in empty seats. Those still have 12 minutes of oxygen left. He probably sat it those seats until 12 minutes was up and the oxygen ran out, then jumped to the next seat and waited again. Because the plane was still flying perfectly and maybe its not protocol to try and contact the pilots if the plane is flying fine. Or maybe it is but he just wasnt thinking clearly and was just thinking about finding that next mask so he didnt pass out as well.

Maybe he did this multiple times, eating up valuable time. Until he finally realizes hes out of masks and has to find more oxygen, which was up near the cockpit. So he gets up there and finds it and then at that moment since hes close to the cockpit decides to try and contact the pilots, gets inside and realizes he cant fly the plane, spends some more time trying to wake the pilots up and maybe looking for a manual or whatever. Tick tock tick tock...fuel running low.

Once he realizes hes screwed and cant fly the plane he probably went and sat by his girlfriend to see if he could wake her up. After his failed attempts and out of desperation decides he will go into the cockpit one last time to see if he can fix it, but its too late now....the plane is out of fuel. Engines flame out...Now hes in a steep dive. Probably just sat there and jerked on the stick and started pressing random buttons. Nothing worked....and boom. Dead.

But who knows really. Sure does seem strange he didnt immediately run up to the cockpit and try to make sure they were OK. From the voice recorder it was said that they heard the door code button noises well into the flight. Almost near the end. They didnt hear any door noises before that.