It seems you don't have much choice except to tell your boss the truth
and tell him that is why you don't want to work with this guy.
Don't talk as if you are whining about it, talk with some confidence and let
your boss know that you are there to learn and to do a good job but you believe
this guy is preventing that from happening.
I don't know if there is much else you can do and who knows maybe your boss
has had these issues with this guy in the past, it may surprise you.
What ever you choose to do I wish lots of luck and I hope it all goes well.