The mockingbird might be looking for a mate. If so, he should stop when he finds one. He could be a younger male and considering how territorial mockingbirds are, he might have to look longer and later rather than try to fend off the older guys -- or maybe he is just able to stay awake late.
(photo1902, did the mockingbird in your yard stop the nocturnal serenades after he could expect some babies? The male helps to build the nest.)
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I know nobody asked, but......the mockingbird was Thomas Jefferson's favorite bird. (Mine, too.) He wrote about them and had a pet mockingbird named Dick. If anybody wants to know more about what Jefferson had to say about mockingbirds, here is some information from the archives......
Mockingbirds | Thomas Jefferson's Monticello