When you pass them flex his or her head in so that she or he can’t see it. Keep tapping him with your leg and half halting. Keep him busy. You could even sing or hum to him or her so that he focuses on you not at whatever he or she was looking at. My old Galloway Sam used to spook A LOT and this method seemed to work for me. I hope it works for you
Don't expect him to spook or tense up, let him walk on a long reign around the corners, if he spooks sit quietly until he settles. Let him naturally get his confidence up. He may spoil every time you walk by but if you stay calm and let stay relaxed he will be more likely to relax. Maybe try tacking him up near where he spooks or just leading him around there or riding around so he learns to relax and not expect tension or a fright.