Amen.You need to get someone out ASAP. You said he is trying to get up but can't. Spooking him and dragging him out isn't going to work. You need to find out why he isnt getting up.
You've said it all, Farmpony.One thing you should know is that if you took this horse in as a rescue and brought it back to health and gave it love and care over the last year, then you did that horse the biggest favor and it died knowing it was loved. That is what matters.