Arrive at the farmhouse, find there was no answer or response. Notice his father through the kitchen window. Run to the cottages and ask somebody to call the police.
Or, depending on the morning routines at the farm, allow somebody else to raise the alarm. My understanding is that Jeremy was typically not the first to arrive.
True, in some variations of this scenario he would be the first to discover the bodies, but he would make sure he goes at a time when he is seen and/or heard arriving, and the farmhouse would be secured. A different situation to him ringing the police in the early hours of the morning claiming to have received a phone call.