You basically have to stop riding the bike when he starts to bark.
Then start just being near the bike and reward for being calm then slowly build it from there, only riding it for a short time, seconds, reward for being calm then build up duration and repeat for a few years :)
I have similar problem with my arrow finding Lab, she gets so exited when doing archery, if people take to long to shoot she will bark and sometimes just drawing the bow will set her off.
When at the visitor centre at Sherwood forest she was barking at the Robin Hood statue as it is Robin with a drawn bow thats how much she loves bows and arrows lol