I'd be inclined to leave him out in the paddock in winter so that he can move around (better for circulation) and graze with head all the way down (better for respiratory system). Better to have quality of life for as long as possible than just life. I'd save stables for injuries or periods of illness where containment is recommended by the vet. If he's really not responding to feeds and the vet says he's not keeping adequately warm then you'll be closer to the kind decision to make. Then again, I'm disinclined to keep horses or dogs alive just because I have the means to prolong life, at the expense of the kind of life they'd like to lead.
We have an elderly horse who won't wear a rug and he hates them. So, as a black horse, he suffered without a cotton to deflect the heat of last week...but he would have run around to the point of exhaustion to rid himself of the rug had I put one on! So, he'll be rugless till he can't get about and keep warm/cool anymore. And definitely paddocked, where he's happiest.