I'm so sorry, Joanna. The aim of the hydromorphone would be to make his breathing more shallow and depress the coughing reflex so he does not need to cough as much. However, it does act centrally and can reduce his ability to take in enough O2, and the buildup of secretions will likely continue.
Depending on the dosage, he would be in effect sedated but it is short-acting so the dosage can be titrated, hopefully, to a sweet spot where he is more comfortable but aware. However, his response is not predictable at this stage, and he may not have much reserve left, so I would make your peace together before that or any other more sedating dosing regimen is initiated.
Many PALS can live for months to years on 24/7 BiPAP, but not being able to clear secretions would mean that ultimately the BiPAP would not be enough to help him initiate and complete his own breaths to the extent needed. I believe you have previously said that he does not want a trach.
Have they titrated the BiPAP to a set IPAP/EPAP (probably low PS, fixed pressure control instead of volume control)/completely triggered breaths that synch with his effort/titrated AVAPS rate? That might help with comfort/coughing also.
To answer your question, I am guessing this is at least the beginning of the end, so I would have that conversation with him and his docs, to reinforce that comfort is the primary goal at this stage.
--Laurie