Has anyone got any tips to stop your horse from rearing just because of the slightest thing? He is not in pain, just if anything at all makes him nervous it’s his first move.

All you can really do is push him forward and try to keep him focused.
Keep calm, don't pull back and when he rears kick then stay calm.
My mare has this same habit. To prevent her from doing this, I close my legs on her and push her forward. When she goes forward, I make sure that my outside rein has a firm contact with her mouth and I play the with inside fingers to get her neck to relax. Hope this helps answer your question!
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