But since you have to buy long stem forage (AKA-forage subst) get the cubes. Start adding soaked cubes and yes if he is an 1100lb horse he will need at least 3 lbs (dry weight of cubes).
After that if he needs more you can use forage stretcher pellets such as your hay pellets if you prefer. But your buying cubes anyhow...meh....your choice if you wish to do both.
Will the barn feed him his soaked cubes on days you are not there?