If his lips won't seal enough to hold the mouthpiece long/well enough to get a breath, he would need to hold the mouthpiece in place for a breath with his hands, if he can do that. Or if he has enough head movement and can open his lips enough, he could sort of perch his lips on the mouthpiece and breathe, if the tubing is held stiffly in place (easy enough to rig).
If that's possible, buy a 22mm mouthpiece from a medical supply store and replace the mask with it -- you can hold it or tape it somewhere initially absent a more permanent mount, and you'll soon know if it will work. They come in slightly different shapes, too, and one may work better than another.
All that said, do we have reason to believe sip 'n' puff will actually help him breathe better? Is this more settings than needing a mouthpiece? Oh -- I see -- this is for hours when he hasn't been using the Trilogy. Never mind.