try a feed called sumo its about £15 a bag great stuff used it on a toast rack tb in summer and was amazed at the transformation and makes coat super shiny.
Ad-lib hay. The usual reason for dropping weight at this time of year is using energy to keep warm. hay is the best thing to keep your horse warm - from the inside - which allows them to maintain - or even gain - weight.
I also recommend the use of sugarbeet.