Assuming you check his environment for ‘scary monsters’, smells, sounds, etc. A couple of things to consider. It is hard on horses emotionally to move. Moving them as little as possible to new environments is wise. If you have a good bond with your horse (trust), taking an article of clothing you have worn several days in a row (like a t-shirt) and tying it in his stall can help. The stinkier the better – as long as it smells like you. I’ve seen nervous horses in new environments stand with their nose in the shirt – it clearly makes them feel better. The next thing is that even though your horse cannot exercise to dispel those nervous feelings, you can do something with him every day. If he can move around at all, do some sedentary ground work, preferably twice a day. Spend time grooming him. If he can do so safely, take him for a slow walk. Establish a regular routine in the new environment – arrive around the same time every day. He’ll quickly look forward to your visits.