Some patients have had trachs. My husband Fred had the stridor breathing early in his diagnose and a trach may of helped , but he wouldn’t agree with one. Unlike your stepson, Fred didn’t want to be resuscitated if his heart had stop.
Have they done a swallowing test to see if a feeding tube is needed? These test show how they are swallowing and if a feeding tube is needed or what type of food is the best for them to have ( thickner etc). If they haven’t done one, I would suggest asking his doctor about having one done.
My husband also had a hard time coughing up mucus. Some use a suction machine for the mucus that sits in the mouth & throat . I’ve heard that some patient use mucinex to help with break the mucus. This is something to speak with a doctor about.
You may want to ask if they can show you how to do chest percussion. This is a lightly tap on the chest and back. The tapping loosens the mucus in the lungs and may help him to get the mucus up.
These are just a few things that may help.