My DS used to do the exact same thing. I know it is hard, but I let him experiement, I made sure that I was close at hand and if he needed my help I was right there.
He would shove a huge piece of toast / biscuit etc into his mouth, and if it went to the back of his throat and he would gag and then spit it out. I too panicked, but he soon realised that it was much easier to bit things than to shove it in his mouth. Don't get me wrong, he still does it if he thinks "Quick get that in my mouth so I can get more".
Now I give him cheerios - not sure if you know what this is - but he bites them in half before he puts them in his mouth, it is too funny.
Just wanted to let you know you are not alone, and send you lots of hugs, I know how hard it is.