You can’t reduce the noise but you can get him used to it by desensitiseing with the noises by creating noise while your doing ground work with him. I hope this makes sense.
Rythm beads around his neck. Or bells
The Le Mieux acoustic ear bonnets are brilliant
There is a ear-net called Soundless, you should check it out.
Earplugs exist for horses (for racing for example). You can put cotton or a ear-net.