Advisor wants first author. Do I have any leverage?
Although Buffy's advice to let this go is practical, and I agree that would be perfectly fine, I think you have a second option here. Namely, if you are careful, you can bring up your concern with the advisor. However, if you do this, you shouldn't phrase it as an accusation or a claim that someone else should be first author; that would be unnecessarily antagonistic and probably would not help your case.

Instead, you could phrase it as a question. Say that you would like to understand how authorship order is determined better. Although you assume that the author order has already been agreed upon for this particular paper, as a starting researcher you are curious about the etiquette of authorship. You can ask if it is common for the PI to be first author in your area. Depending on how receptive he is, you can ask other questions.

If you follow this approach, then probably one of two things will happen:


First, he could become defensive and dismissive. This is more likely if he is really being unethical. In this case, Buffy is probably right that your best bet is to let this go for now. Sadly, these things happen; author order isn't always determined correctly, and that can be either due to honest error or due to dishonesty. You could always talk to another professor or mentor for moral support and to assess how to proceed (particularly for future projects), but probably for this one accepting the order is not such a big deal in the grand scheme of things.
Second, he could be helpful and receptive. Maybe he will explain that he is usually first author on his papers. Perhaps he has a good reason for this. For example, in some fields, that is accepted, while in others that is pretty condemned (I don't know what is the case for your field). But he could also have some personal justification for why to go against the convention. I would not agree with him, but perhaps if he explains this then it would make it easier for you to understand.


Whatever you decide, try to see the bigger picture that authorship is not always obvious, and this professor will be judged for his own actions by colleagues and the community in the future; it's to some extent out of your hands. Good luck.