Why panic is information, not naughtiness
A horse is a herd prey animal, and separation alarm is wired in deep: alone, in his wiring, means exposed to whatever hunts horses. When a herd-bound horse calls, paces the fence line, sweats and refuses food, he is not staging a protest — he is genuinely frightened. That is why the old approach of leaving him to 'scream it out' so often backfires: a horse who panics for hours learns that being alone is exactly as terrible as he feared, and a horse who throws himself at a gate or fence can injure himself badly in the process. Graded practice works with the wiring instead of against it, teaching him in small, survivable doses that horses leave, horses return, and nothing bad happens in between.
Grade the distance and the clock
Build a ladder of small stages and only climb when the current rung is boring. Start with him eating his feed alone while the others are visible across the yard. Then lead him a short way out of sight and straight back. Then stable him alone, hay net in front of him, while his companions are still on the yard. Then stable him while they are led away briefly, building from a couple of minutes upward. Each stage gets repeated on several different days before you lengthen it, and if a stage goes badly you simply drop back one rung for a few days — that is the plan working, not failing. Distance and duration are separate dials; turn one at a time, never both together.
Make alone time pay
Alone should become the place where good things happen, not the place where the herd vanishes. His favourite feed, a generous hay net, a scratch in the spot he loves — all of these belong in the alone setting and ideally only there. Rotate which companion he is separated from and which he is left with, too, because a horse bonded to one particular individual needs to learn that horses in general come and go, not memorise a single partner. If he will eat, he is coping; a horse too stressed to touch his hay net is telling you the step was too big, however small it looked on paper.
Practise in both directions
Being taken away from the herd and being left behind by it are two different skills, and many horses can do one long before the other. Practise both: sometimes he is the one led out of the field while the others stay, sometimes his fieldmate is led away and he remains with you, calmly held or grazing in hand nearby. Also practise leaving the yard and returning with nothing attached to the outing — no event, no lesson, no vet — so that departures stop predicting a whole demanding day. Short, frequent, undramatic repetitions are what rebuild his confidence; the occasional heroic outing mostly rebuilds his alarm.
Know the signs you have gone too fast
Calling that escalates rather than settles, fence-walking, a soaked sweat in cool weather, dung that turns loose, and any attempt at the gate or fence mean the stage was too much — end the session as calmly as you can and repeat the previous stage for a few days. A horse who actually injures himself trying to get back to the herd, or whose panic is extreme and not softening over weeks of careful grading, needs a vet involved in the conversation, because pain and genuine anxiety conditions sit beyond what practice alone should be asked to fix. Keeping the ladder on record matters too: with the stages written into KinsLe's Care Teams, the next person on the yard can see at a glance what he coped with yesterday and what today's session should ask of him.