Some questions for you to ask:
Is his coat adequate? Some horses in the winter look as though they are still sporting a summer coat and some look like decendents of the wooly mammoth. Is his coat long and fluffy or short and close to his body?
Second, does he have constant access to hay? Does he have a round bale to eat from all day? Or does he get two flakes tossed out in the morning and once it's gone, that's it for the day? If he has hay to eat all day, like a round bale or something similar, he will stay pretty warm just from the heat he creates eating and digesting. If he doesn't have constant access to forage, he could be burning calories to stay warm which may cause weight loss.
Third, does he have access to shelter? Not, does he use it? But does he have access? If the winds and snow are bad enough that he wants to go into shelter, but has none, then he may be cold. But if he has access to shelter and doesn't use it, it's likely he's not as cold as you think he is.
Now, this is just my point of view...but I've never blanketed my horse over the winter, he goes out rain, shine, wind, sleet, whatever. BUT he has a round bale to stick his head in all day and a shelter to go to if he chooses.

Horses do fine in cold if they are getting fed enough. Unless it is getting below 25 degrees, I would not worry about him. Other factors can be how old the horse is, if it is over 27 or so then it may need a cover or shelter and extra grain or other feed. Wind chill can be a factor. If it is below 32, then making sure fresh water is available and maybe a warmer for the water to encourage drinking and prevent freezing. So keep your horse fed good quality hay at least two or three times a day, good fresh water and a wind or rain block and it should be fine. In the wild, horse will use trees and terrain for wind blocks and shelter. It you trim or shave your horse, then everything changes and if you don't blanket him, I would say you are abusing your horse. If you are leaving him natural and feeding him well, he should do fine. Hope this helps. Rick

Sorry...not sure WHY that posted twice, must have used a 'back' arrow at some point. Sorry!

Hey Guys,
Thanks so much for the tips. I have never shaved and would never shave my horse. I live in Steamboat, where it can at times get below thirty degrees. However, this weather is extreme and unlikely. He was born and raised in steamboat. However, when i bought him, his owner had a barn and put him in at night. He has a very nice thick coat and is used to the winter. He has access to a shelter, and the lady i board him with gives them a lot of hay and grain at night, esspecially when it is cold. I have had him for five years and never really thought about blanketing him, but i was really just checking because i was noticing how cold it got all of a sudden over night. Also, his water trough has a heater, although it doesn't make the water warm it does keep it from getting too cold or freezing. So, I guess I am doing nothing wrong in the way of keeping my horse warm or not. I did assume that becasue he has a thick coat and shelter available he didn't need a blanket, but then I thought back to how he used to be kept in a barn at night before I owned him and got to wondering. But from what you guys have told me, he is just fine. Sometimes I just worry about him because I am not able to visit him all that often. Anyway, Thanks so much for your help!!!

If your horse doesn't usually have a blanket, its not a big worry. But since the night time gets colder you could put a stable blanket on him at night. When you start full time blanketing its a hard and long time to get them off it. I made that mistake and now my horse has to have a winter blanket on 24/7. But if he is eating enough i woundn't worry to much about it.

you could maybe out a sheet on him if you knew it was going to be REALLY cold

I wouldn't recomend putting a light sheet on a horse in the winter, unless your putting a thick rug over it! When a horse gets cold his hair stands up and traps warm air acting kinda like an insulator so if you put only a light sheet on him his hair will not be able to stand and therefore impeeding his natural insulation!

Very good Alison! I would agree 100% with that one! Kinda makes matters worse.

I am believer either choose to blanket or not at all... I agree with Allison that putting a sheet on in mid winter is going to be harder for the animal to adjust. I blanket my horses becuase they work all winter long (and one is a 3 yr old TB with NO coat). If your horse has been fine in previous years with out blankets I would keep him that way. There is a lot of maintenace and cost to blanketing (buying, repairing, changing, washing, etc....). Horses will naturally adjust to the drop in temperature. As long as they have shelter and lots of hay they should be good to go!
If you are schooling all winter long in an indoor and they will be sweating, then banketing and/or clipping will help them from getting a chill in the winter.