RE: Got a deer...need help...
Fields can be touchy as far as marking the true spot where the deer was hit...it's not uncommon to misjudge it by many yards. Did you have a bush, a tree within the woodline, or some other identifiable marker? If so, mark it and begin there, regardless of presence of blood/sign. If not, try to recreate the sightline you had and have your brother mark the nearest spot you believe to be valid.
His reaction to the shot, of course, indicates a hit but such a reaction can easily be misinterpreted.
The rain is unfortunate, but I probably would have tried to stay on him before any fresh blood would have been washed away by morning (even through the night). Sometimes, a deer dog (where legal) on a leash can really help out in such situations.
Very disheartening I'm sure but you may well find him closer than you think...I hope so.
GL Koon...