How old is the horse? Age can be a factor in keeping some horses healthy and at good weight.
How long since the horse's teeth have been checked?
Have fecals been done in the last few months to learn if the horse has a worm issue?
Metabolic issues can make a horse lose weight instead of gain weight
I have one of each.
When the vet did physicals in October, he said to not let the horse with Equine Metabolic Syndrome lose anymore weight, going into winter. I have been able to get some weight back on him but it's been carefully managed due to his insulin issues; part of the management is feeding him three times daily.
My point is, get the vet involved and go from there