Personally I don't think there is many options here, if it doesn't have any obvious injuries then it is very likely going to be a window strike, and that means the injuries are internal, and it is likely concussed. I don't think a rehabber (or anything else) can help in a situation like this, just have to hope it's not as bad as it looks! If it's warm you could try leaving it in a protected/sheltered area outside like a bush, but if it's nasty out then it might be better to keep it in a dark box, and see if it perks up. At this point it might just be giving this poor guy the most peaceful environment to pass away in, unfortunately! Thanks for looking out for your local wildlife!