Spooky or not, some horses just don't like things on your ears. If a friend has one you can borrow, try that first. Show it to your horse, let him/her sniff it, keep it in their line of vision as much as possible and slowly move it closer and closer to the ears. If your horse jerks or gets anxious, take it back to square one and let them look at it some more. My friend's mare was fine putting it on but didn't like the feel of it on her ears, which is why I suggest borrowing a bonnet if you can so you don't waste money on something your horse won't wear.
Put ear balls/plugs in first, try him in those for a bit, then move to an ear bomnet
Prepare him. By desensitising him to the ear bonnet and ensuring he is perfectly comfortable with it around his face, he shouldn't spook at it or feel uncomfortable wearing it in future.
Start by allowing him to sniff and investigate it. As his confidence grows, rub it on his shoulder, neck and eventually lead it up to his face and ears where it will be. If he shows any discomfort by attempting to shy away from the ear bonnet, stay with him, keep the bonnet on him until he stands still. Only when he stands quietly should you take away the bonnet to 'release' the pressure.
Hope this helps and good luck!
If you have friends at your stable with a bonnet, ask them to lend it to you just to try! If that option isnt available well I'd buy it
If its his first one, make sure it doesnt have tassles so it doesnt spook him