For one, it's very,very shallow and looks as if it's fallen. It does not look as if it's been pulled over. Why would you pull it over so that it fell on the crop area? If you were going to pull it over you'd pull along the unploughed strip.
Two, the packing stones would scatter with it being so shallow. Also, some of them could just be field clearance, thrown around the stone because he doesn't plough there. It doesn't look deep enough for there to have been packing stones in place. If they were there they weren't very firmly in the ground anymore.
Three, I'm sure the farmer knows that it's old ... that's why he's been ploughing round it. Whether he cares or not is a different matter and one only he knows the answer to.
Four, the tent pegs are a good idea!
Getting in touch with the county archaeo should probably be the first move. Let them speak to the farmer about standing it up. A bit of prelim excavation should be done before it's re-erected.