How can I make a night scene black and white while keeping only the subject in color?

One option for blending/fusing two shots would be to take two exposures (ideally using a tripod), one with flash/spot on the man (and otherwise heavily underexposed), one without the flash/spot (exposed for the background, keeping the man still + dark to avoid layering colour over B&W).

You'd have to be careful to control the lighting in the area around the man, probably with off-camera lighting with 'barn-doors' or similar to really control the light spread and create a harsh shadow where you want the 'colour' shot to end.

If you need to take the shots in quick succession (e.g. if you need the man to be black in one shot and lit well in the other but otherwise in the same pose) you could use strobes that take a little time to recharge, so they simply fail to fire in the second (B&W background) shot so as to avoid illuminating the man.

No need to adjust the scene in the camera between shots, you can easily convert the background shot to B&W in post.

Then you're just adding the two shots together (the coloured section + blacked out section = colour only, the black background of the colour image + the B&W image lit up sections = B&W image only).