In my experience (42 years hunting), the fact that he got to where it layed down (in the dark) is a sign it was hit reasonably hard. If I was to bet I’d say that deer layed down again as soon as it felt safe, which would be beyond the point where they gave up. That may well be closer than you think and possibly within a couple hundred yards. Walking a grid pattern looking for a deer might work. All of this is naturally dependent on the wound type...vascular injuries like leg or neck wounds bleed a lot but may never kill a deer. Wounds to the chest cavity, where air is getting into the chestand messing with the internal,pressures, make it very difficult for a deer to breath. In those cases they will lay down quicker and is very hard for them to recover from. Liver is fatal as well, but can take a while, but often the deer lays down fairly quick. Finally, watch for crows and/or buzzards...they circle quickly and if it’s cold and the deer hasn’t been eaten they may lead you right to it.