My DS used to do that with currant buns, which are untoasted bread so similar thing. It used to get gooey and I used to have to scrap it off the roof of his mouth with a finger, when he started to struggle. On one occasion he ended up retching so hard it made him sick, but frequently gagged on other occasion.
Difficult though - I ended up not giving DS currant buns until he was old enough not to stuff his mouth full. Didn't seem to happen with bread though but we don't usually have sliced bread which I find gets a bit stodgy in the mouth if by itself, even for me.
Anyway, to get to the point, I don't think it is that unusual. I know with BLW it is all about letting them feed themselves but perhaps, just for a while, you will have to limit how much is available to him until he learns not to stuff so much in.