I'm confused. You state that you feel like he uses you "in his lab to carry out experiments
for him" which is precisely what you're there for. What else are you looking for in lab? How long have you been there? How exactly does your PI think you're stupid? You also mentions he knows your grades. I'm not sure what this means. Grades usually don't mean much in the lab, it's a different kind of process than studying and taking tests.
Furthermore, you state 'none of this matters' once you get into med school. Why are you even in graduate school then? Why are you working in a lab if that's not what you want? Perhaps your PI senses/knows this and you take it as his thinking you're stupid.