So I put him in the corral with a calf with our trainer there. He broke his leg trying to jump out the of pen.
Desensitizing a scared animal requires you keep the animal below its "freakout" threshold while exposing it to the scary thing, and then gradually, oh so gradually, try to raise the threshold of comfort.
Trying to ride this horse past the cows and through the bikes is "flooding", and I suspect no matter how many times you do it, the horse will be afraid, and possibly get more and more afraid- fear and anticipation of fear begets greater fear.
So you'll have to try some other, more gradual approach. With the bikes, getting someone to ride a bike far away from the horse at first, and then gradually getting closer, can be easily done.
I'm not sure how the area appears vis a vis the cows, but perhaps you could hand-graze the horse where it can sort of see the cows but isn't too close, and slowly, oh so slowly, move closer?