In my (admittedly limited) experience, if a supervisor asks the same question repeatedly, it simply means that you haven't given the "right" answer the first time around and they are waiting for you to spot your own mistake. Quite a passive-aggressive tactic, I know, but unfortunately quite common.
You are in this program because you wanted to get a PhD, right? I think that switching to law school is a step too far; it might be just a temporary hiccup. Switching PIs is not as uncommon as people think, but at the same time it means that a lot of work has been wasted. It's also not a great thing to have on your CV; lots of future employers in academia might look askance at that sort of thing.
It's not going to be a popular opinion on here, but I would say that you should stick to the lab, beg him to let you finish the PhD, and in the meantime generate data at a furious rate. If he's adamant, maybe you could talk to other members of your advisory committee (if you have a good relationship with them and they can be relied upon not to tell tales). Perhaps a chat with a friendly postdoc might help too.