Winter Aged Bone

at my aunts barn where we board.. my grandma has a 20 year old ex. thoroughbred. as he is getting older, we are having trouble keeping weight on him and you are now somewhat able to see his hip bones and ribs.. i was thinking for winter, i would just get him , like, a winter sheet or thin blanket, as he gets fuzzy in the winter but just for extra warmth.. what kind of blanket have you used that is not REALLY expensive , but will do the trick
p.s. he is well fed, and and cared for amazingly, he is just getting to that age
please no rude comments.