Is he using a nasal, or full face mask? and what's his pressure at?
A couple thoughts come to mind. If he's using a nasal mask, he might be mouth breathing, which could make treatment uncomfortable. If he's using a full face mask, it might be leaking which could also make treatment uncomfortable as well.
If he's having central apneas, he might need to have another sleep study to have his pressure adjusted.
Also, we breathe based on the amount of carbon dioxide (C02) in our blood. Bi-level treatment helps you breathe deeper, which means you get rid of more carbon dioxide. When this happens, your brain tells you not to breathe until C02 levels go up; thus causing central apneas.
Therefore, if his pressure isn't too high, and his mask is fitting good, he might do better on CPAP.