I'm sorry you are having problems with this chick.
I can't say that I have ever had a chick that didn't visually show a crop or that seemed to not be well developed internally.
If the tiny fellow has a blockage of food in the oesophagus/throat, then that must be why you are unable to feed it properly.
Most likely the food previously given was too thick in consistency and that resulted in the obstruction. When the mother budgie runs out of crop milk to feed the youngest chicks, they can at times feed them the whole seeds that are normally given to the oldest siblings and this will of course not sit well with the youngest chicks who are not yet ready to accept this food and will suffer from a serious blockage that can cause their untimely passing.
Given how serious and extremely delicate this is, your chick needs emergency assistance from a qualified and experienced professional.
Depending on the situation, it may be possible to remove the obstruction by flushing out the stuck food.
I'm sending my prayers for the very best outcome for your little one.