My dog has a spot on his neck he really likes scratched that is as effective as treats, so you would have to find something like that for your horse.
However, you can teach your horse to be polite about treats using clicker training (which I assume you are using to do the trick training). Simply DON’T give him the treat when he is rude, and click and give it to him when he is being good.
If he is pushing at you and nuzzling at you, no treat.
The second he moves his head AWAY or withdraws a bit, click and treat.
Think of it as teaching him the trick to “wait politely.”