My manager wants to be the lead author for a paper which I have done all the work for. What ethical reasoning should I use to dispute this?
I feel confident in my answer because, although norms differ between fields, you are in exactly my field.

You are absolutely correct that it is not ethical for your manager to ask for first authorship on this paper. From the work you describe, this is your paper. He may need more first authorships, but so do you, and even aside from the ethics of relative contributions, his role should be to help you to progress your career. 

Managing this situation is going to be very tricky for you. This happened to me during my PhD -- my PhD advisor was first author on all my publications -- and I did not confront him over it. Obviously, this wasn't good for my career. It wasn't great for his career either, because it meant I was unwilling to work with him after I finished my PhD and so he lost the opportunity to be a coauthor on several more papers that followed on from my PhD work.

On the other hand, having a blow-out with your boss that puts your ongoing work in jeopardy or results in your work not getting published at all is also not going to be great for your career.

I suggest you do try to change his mind, but keep it very polite and civil. Mention the benefits to him of having you do well: it will boost his reputation as a successful PI and mentor, it will increase the likely success of future collaborative work with you, and it will reduce the amount of work he has to do on the paper himself so that he can concentrate on getting more of his own work published. If this fails, it may be worth involving a third party -- in the first instance someone you mutually like and respect, if possible, and not his boss -- to facilitate a discussion about setting some more formal guidelines around how authorship order should be determined. 

If all else fails, you could appeal to authority - his boss or the journal editor - but be warned that WILL get messy and bitter.