About feeding a horse?

I am doing an ignment and need some info on feeding horses. I am a dog person but have been asked in my ignment what advice I would give to a client (Im training to be a vet nurse) regarding the dietry requirments (type of food frequencey of feedinding and justifying choice of food) of a 27 yr old thouroughbred gelding horse retired in a paddock during a drought. He is considered to be underweight for his size. Thanks any info will be helpful