Looks like you've been getting some good tips. It definitely wouldn't hurt to get your vet involved and at least ask what to do. I have a 30 year old Tennessee Walker and I worry about him in the winter. I started feeding him soaked beet pulp along with his Senior feed last winter. You really can't overfeed it since it falls more into the forage class instead of grain. You want to start by keeping them warm from the inside out and hay is what you would normally do that with, but my guy can't eat hay anymore so soaked beet bulp it is. And in the winter you can use warm water to soak which helps. I also add 1/2 cup of vegetable oil to each feeding for extra calories. And then if you're not already, definitely blanket him. They burn a lot of calories just to keep warm so blanketing can help them out with that. I used to not believe in blanketing much but my senior changed my mind. :) Finally, and this isn't an option for many since not everyone has their own property, but we planted winter rye on our fields last winter since hay was sooooooo sparse and expensive last year, and wow, what a difference it made in keeping weight on my crew. There's nothing better than fresh green grass in the winter. It did wonders for our old man. But like I said, if that's not an option, the other things I mentioned in addition to what the previous folks suggested might help turn things around for ya. Good luck!