My mare used to be DEATHLY afraid of motorcycles! My quiet trail horse would transform into a bucking bronc trying to run away from the loud thing that scared her so badly.
For my own safety, (since road riding is a essential to get to the trails) I started to dismount when I heard or saw a motorcycle approaching, and I would hold Indie as it passed.... at first she still freaked out, but I was better able to control her from the ground. After awhile of doing this, I started to let her graze everytime the bikes would pass. It got her to start putting the bikes together with yummy grass... and since the bikes never hurt her and I was calm she slowly put it together that there was no reason to spook.
If you have a pasture that is next to the road (I use my front yard), putting your boy in there for a few afternoons will help a lot too. As the bikes pass, they will start to become quite normal, and he'll learn they're not scary. Pasturing next to the road has desensitized my mare from school buses, trailers with equipment and flapping tarps, and yes motorcycles.
Hope that helps some, and be careful! Falling off a horse and onto pavement can end badly real quick... better to dismount if you have too than risk a bad fall.