How to help winter coat growth??

Okay so I have never had a problem out of any previous horses or Scout when it comes to this time of year and their winter coat coming in. It has just started to get chilly out and we had our first major rain and it was sooo cold. I have yet to get my winter horse blankets in. Also, I am not sure our horse shelter will be finished this year, therefore the horses need their winter coats.

Scout has already started growing his, so he is a bit fuzzy but Halo on the other hand still has a very short coat. When it was raining the other day, they stayed dry because they were on our front porch, haha, BUT Halo was shivering severly. I don't want to have to rely 100% on a blanket because I do want her to get some winter fuzz. Is there anything I can do to help her get it? Any kind of food or anything, lol?

Here is a picture of Scout and Halo just from yesterday and you can tell the difference in their coats. Scout is starting to get fluffy but Halo is still pretty slick.

I would at least cover him with a waterproof uninsulated sheet. If he's dry, he can keep himself warm with a minimal coat, but if he's wet, there's not much he can do for himself. If you put an insulated blanket on him, his coat will stop growing in for the most part., so just keep him as dry as possible and his body should take care of the rest.