Hello, my name is***** and I have over 20 years of experience as a veterinarian. I understand that you are concerned about a bunny being outdoors overnight in 51F temperatures.
Bunnies are more adaptable than we give then credit for.
If your bunny is otherwise healthy, eating a healthy diet well and drinking well, has been living outdoors for a while and has been through temperature fluctuations, and has a hutch that has a sheltered area that doesn't allow wind into the sleeping area with plenty of loose bedding to burrow in then I don't see a problem with your bunny staying out.
The concern for bunnies is a sudden change in environment. Bunnies used to being indoors with little change in temperature, and not used to knowing how to burrow into bedding out of the cold and wind, or that are sick with other stressors won't do well with abrupt changes in temperature, and are much more likely to get sick.
So if he is just learning to live outdoors, or doesn't have a sheltered place to sleep with lots of bedding then you may want to bring him in if this is a drastic change in temperature.
Please let me know if you have any additional questions.