This really bothers me - it seems to me that he doesn't respect me and kind of sees medical students as lab slaves that will just do whatever he wants us to do. I mean, common courtesy is to notify me ahead of time and see if I'm still interested in this project correct?
What would give you the opinion that you are anything other than a lab slave? You don't have funding. You aren't an expert in your field. You don't even have any clinical experience yet. You should consider yourself lucky that you have something to work on at all.
I don't mean to sound harsh, but I'm a board-certified neurologist, with independent post-doctoral funding, and I'm still somewhat of a slave to my PI. He gets the funding and introduces me to new contacts. I do the lab work and write papers and grants. We both get what we want out of the relationship. What does your PI get out of his relationship with you?
Tell your PI that you were really excited about your other project, and ask if maybe you could split time between the two. Don't leave. Grin and bear it. I imagine your PI probably has a reason for asking you to work on this other project. Maybe he thinks it is more likely to be successful than your prior one.