Mine gets a handful of mollases free hifi with about an eighth of a scoop of sugar beet in because the others all get something and I feel cruel otherwise.
one of my youngsters gets just under half scoop of D&H mare and young stock and 1 scoop of alfalfa and as a treat gets warm bran poss once /twice a week to warm he up as she is very fine and lost weight this winter. the others are little fattys and get hay and bran as a treat once/twice as before. no hard feed.
if not underweight i would just feed hay but if he starts loosing condition would recommend D&h mare and youngstock
some alpha a/and or high fibre cubes and a bit of sloppy sugar beet. then at least you know his belly is lined, and he has fibre and moisture in his gut. as he has grass and haylage i would just feed a small maintenance ration, and it gives you time to check him properly and keeps him humanised.