Does he have a good level of fitness?
Does he do sprints and other pitching-specific exercises?
For bullpens, you want to build up the volume gradually.
How many pitches does he throw in his bullpen? If he's expected to throw 70-80-100 pitches in a game, then you want him to be able to throw that many in the bullpen. Otherwise, how are you going to make up 20-30-50 pitches?
He's gotta be conditioned to throw the amount required of him.
But that doesn't mean make him throw 80 pitches if he's only been throwing 40-50, you want to build the volume gradually.
As I said above,
To get better at pitching you have to pitch at game intensity, perferably in blocked sets, as it makes no sense to throw a 60, 80, 100 pitch bullpen in 10 minutes.
So how often should he pitch?
Twice a week would be nice (excluding game).
Get him to throw the amount of pitches he's required to in a game.
Thus, if they expect him to throw 100 pitches, he better be throwing 100 pitches in a bullpen.