Oh dear, Sarah! Like Simon - I am so sorry to hear you have had to come back on here - after all you went through the first time!! And I can see that having been through it with one son - doesn't necessarily help when it hits again.
But you actually do have the skills and know-how to try and tackle some of the issues. Remember you were in the dark when your first son had his first issues - whereas you have travelled many roads and bumps since then.
Like the others have said - there are questions that we hope your son can answer - to give you some idea of why he suddenly feels this way. Although I do recall - this son had a couple of wobbly days or times way back - but it was seen at the time to be due to your other son's troubles?

At his age - going on 16 - it is a bit later than the usual SR, isn't it. I wonder if, like Leah says , it is just the enormous pressure that are put on kids these days. Is he feeling that the expectations from the school and perhaps even yourself are too high? Fear of failure? Better to bail out before sinking etc....?

Hard to know what advice to offer - but i think the others have given you some idea where to start thinking - again! Poor you - this is the last thing you needed. Can your other son talk with him? can he offer any insight - given what he has been through?

Came back any time Sarah - we are all here to help - even if I haven't been around much latelly - due to holidays down here and also moving house.
Take care and let us know how things progress
Linda