I have to disagree. I would try to find a dual ventilation setup to try.
Yeah one of the doctors suggested that, we did try that a couple weeks ago and it didn't help, we didn't have a proper dual ventilation setup so the mask didn't seal as the cannula was in the way. When his CO2 is low he's able to stay above the 90s, even a few days ago when it was high but not too high, as long as I held the mask on when there was a leak he was staying in the 90s but given that this happens desat after the BiPAP is off I feel like there's some other explanation like his lungs are freaking out