Part of the way to successfully deal with a horse that is suddenly spooking at something that was formerly not an issue is for the rider to control the tension in his/her body. If you can stay relaxed when he thinks there is something spooky there, your body language is telling him that there is nothing there to worry about. But if you tense, then he will also feel that and assume you have the same concerns he has, and that there really is something spooky or dangerous. Keeping your body relaxed while the horse is misbehaving or spooking is obviously easier said than done and takes practice. One way to relax is to pretend what he/she is doing is amusing instead of annoying, frustrating, stupid, etc. Sometimes being able to laugh at your horse and yourself can be very useful.