Maybe an idea is to go to Boots or Holland and Barrett (much less expensive!) and check out the books that are there with the remedies. They are talking about people in them, but if you know the horse well and observe his behaviour you can apply it to him. The main ones for this situation to read are Star of Bethlehem for shock of separation, Walnut to help adjust to the situation, Sweet Chestnut for hopelessness and despair and Rescue Remedy to buy anyway because its good for horses in nervous or stressful situations and for people too, so its always good to have handy...
Select the ones or one that sounds appropriate for Merlin and give them to him at least 4 times a day on an apple quarter or in his water (this is less helpful because I have never known a horse that drinks a lot). Do this until he improves. Four times a day is a minimum at the beginning more frequent than that is fine.
I hope it helps, try to do it as soon as you can to help him out.