Welcome to the Forum...
From your avatar picture what a beautiful horse...
We all started down the road of horse and ownership with butterflies in our stomach and lots to learn so you are not alone.
So, is your horse boarded in a barn where the barn supplies and feeds or do you do all yourself or are you at home or some combination of any of those things?
Horses are grazers and have small stomachs so less food fed more often over a period of time works best for them.
Cubes are fine if that is all you can provide but actual hay would be preferable in a grass mix since each type of grass offers different nutrient parts to a diet needed.
Alfalfa is a great food, but rich and many horses don't need such top of the line food fed only...they do best with a variety of grasses, hay and in your case cubes offered.
Is cubes the only thing your horse eats or is there are foodstuffs given? IF so exactly what are they and how much of each?
I saw from your IP you are from the mid-west more westcoast area and know out their it is common alfalfa is what they eat, period.
So...only other part of that to offer is many who feed cubes soak or at least moisten them as being dry they absorb a lot of moisture as they travel the digestive tract starting in the mouth and that can lead to dehydration issues if not addressed carefully, hence soaking for a bit before feeding is done.
All foods of feed, hay, cubes are fed by dry weight not volume with horses.
Hay/cubes are fed to maintain weight at about 2 - 3% of horses weight so a 1000 pound horse would be fed between 20 - 30 pounds of forage products optimally to maintain, in your case they are probably going to gain being fed that much rich a day...
Make sure your horse has more than a adequate supply of fresh clean water when eating and if possible keep an eye to how much he is drinking so you know he gets enough water to hydrate those cubes and not make a blockage from to dry suddenly meets intestinal wet and expansion and a roadblock of the gut happens...water and lots of it, hence soak/moisten those cubes to start and more water always offered is needed.. Welcome to horse ownership and new gray hair arriving.
Could do without the additional gray on my head but horses are here to stay!!