I also would believe it is cold there right now, correct? This slows down the water intake a great deal especially if you get a semi warm day and then a cold night! If you can get ahold of some electrolytes and add those to his water in the winter it always seems to help mine to drink some more water so I do not run into impactions colics. Impactions are generally caused by the fact that the horse ate to much dry matter and not enough water to keep it moist going through the intestinal tract. I would definitely call the vet in the morning like you are planning. Why didn't your BO call when she thought he might be colicing? If I even think anyone's horse who is here is colicing, they get an immediate call from me and then if it is not severe I will wait till they get there and then call the vet and in the meantime I will administer the mineral oil if I believe it to be impactions colic(which is probably what your vet will do to your horse along with banamine or bute).
Horses can be allergic to anything, is the ricebran new to the diet or anything else?
Really dry poop that does not look shiny and wet is NOT OK. That means that there is something slowing down his intestinal tract that needs to be moved through and also that he needs to be drinking more water than what he is