If adding more fat/oil to his diet turns out to be not enough, I'd really suggest blanketing him as well.
Correct blanketing (aka, referring to a blanketing chart and using suitably weighted blankets for outside temperatures) can do wonders for keeping weight on, especially when it comes to older horses.
I know with my mare (26 years old), she starts dropping weight like crazy when it gets cold if she's not blanketed, no matter how much hay/grain she gets. She'll get "hot" from the extra feed but she'll be skinny and "hot". As soon as I blanket her, the weight will stay on and I don't need to feed nearly as much because she's not needing to eat a ton to stay warm.
However, I'd start with trying to change his feed regime and if that doesn't work well enough, then consider blanketing. :)
Fabio - 13 year old Arabian/Lipizzan gelding
Rest peacefully, Lacey.