If the Q-tip doesn't work again, try pulling down his jaw very lightly. When I don't have Q-tips I will start right at the beginning of the lower jaw and slightly/gently pull down the jaw with my thumb and take a look inside.

It could be your substrate you are using, but if not it could be mouth rot but I'm not seeing anything cheesey-looking in that photo.

Some snakes get really uncomfortable while doing the Q-tip thing, so when they do just give them a couple minute break. You have to get them securely behind the head (you should feel two small, round bones in the back of the head) to be able to get it to work for those who are uncomfortable with this method.