It is true horses needs access to hay about 16 hours a day but I thing it shouldn't be stabled full time. It should get at least 5 hours or more out in a field.It should get loads of exercise.

Is there a reason that your horse must be stabled full time, does he ever get to just be a horse and turned out with others for his mental well being? He must be very bored and should have unlimited hay if this is his only pleasure.

Preferable give your horse ad-lib hay to help- prevent boredom and reduce stress in the stable. If he is prone to gaining weight then I would recommend that you soak the hay and/or place it in either a small-holed haynet, or one over anjother. This will make it more difficult for your horwse to eat his hay so it will take longer to eat, meaning that he eats less overall.