Your horse isnāt just āreactiveā. Your horse is a āsentinelā. One that lets the herd know that something is out there, we all need to pay attention. The extra reactivity isnāt needed in every horse in the herd. Just one or a few. So, some horses have it to a greater degree than others.
Second the suggestions above.
I feel for you and the horse. This is not an easy fix ā but maybe it is an opportunity for both of you. Even though it is probably going to slow your initial training as you take the time to address the reactivity.
If it helps, look on it as helping your young horse become as impervious to outside distractions as you can help him be. You will have a much more solid partner as a result. With luck, not much will bother him at any kind of show, in future years. This is the payoff to the extra training time now.
My approach has been that of course the horse is going to notice, as horses do. I canāt look on that as āthe problemā because there is no stopping that. What Iām programming into the horseās mind is āthis is what to do when suddenly hearing a scary noise, or seeing something weirdā. If the excitement comes, the adrenaline spikes ā we calm ourselves, right away. We know that itās ok. Itās not ābeing calmā beyond their individual capacity, itās ābecoming calm in spite of thingsā that counts.
The way I choose to think about this process helps immensely not to become too discouraged when progress seems slow. We are working on how the sentinel responds, over and above the instinctive reactivity.
The result may not be perfect. The horse may still be one that notices things - I honestly think there is a genetic component to stronger āsentinelā horse behavior. Itās how rideable the horse remains that counts, more than that they are a noticing horse.