baby coughing when eating hard foods and water

So my son has been going through this coughing thing since he turned 1. We've done 2 swallowing test and they are saying that he's not chewing his food basically not using the jaw muscle as hard as he should... My thing is when he was about 6 months I used to give him pizza crust and he had no teeth and he never choked so why is this happening now? This is like a never ending story. If I give him pizza crust now or a piece of bacon he is coughing non stop. I'm so frustrated. Has anyone ever gone through this?