let him sniff you before you touch him.
Husband said take him into the field, stand with your back to it, ignore and let the horse come to you. When it comes to you walk away, then after a little while stand still, turn and face it, let the horse come to you. Put your hand out open palm and let it sniff you, slowly touch it on the neck. Every movement has to be slow and if the horse backs off take it back a step. Repeat until the horse is more confident.
Husband has been with horses over 30 years, I just got told off, never trap in a box. The first time you try in the field the horse will run around, just ignore with your back to it.