If what you're calling a biscuit of hay is the same as what I call a flake of hay, your horse can definitely have more hay, especially if he isn't turned out. Ours get free choice hay and keep weight really well. High quality hay is best, I've seen horses with free choice poor hay that looked really skinny absoultely bloom on a better quality forage. I've heard that beet pulp is good for adding weight, but I've never tried it myself. I lace my boy's feed with a few tablespoons of corn oil for added fat after the free choice hay.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown