This is the thing, at this point there is nothing you can do. Usually before I sell anything, I take pictures and maybe even a video saying today is November 14, here are the lights, they are completely functional. And if they complain, then you have proof and you are not liable.
Since you can not prove that they were working when you sent them (Even though I am sure they were), then there is nothing you can do. The best option would be for him to send them back, and you refund him.