Yes, he is definitely very skinny. I'm glad you were able to get him to a vet
One thing: I think you mentioned you're feeding him every four hours including over night? If his crop isn't totally emptying as you said I would not feed him overnight. I only ever feed between 7am-11pm. It is important that the crop totally empties once a day (ie- overnight) so no food is left to go sour.
Perhaps it is something to talk to your vet about though. Since he is so thin he might need the extra nutrition from more feedings, I don't know. But the crop not emptying is worrying.
I am raising some babies right now including two cockatiels and something I noticed about the tiels (these are the first tiels I've raised and they are about a month old now) is that they always act hungry! Even if I've just fed them they are still begging for more. I have to refuse them any more food or I think they'd eat until they exploded lol. They are very good eaters, chunky little babies! Cute little brats