Yvonne, unfortunitly ive seen it alot
it is sad...sometimes its not that the owner isent feeding him enought its that hes feedig him to much of the wrong stuff...Cockatiels should be kept on a pellet based diet but make sure to offer them seeds too...any variety seed mix for tiels is good, Hagen is one of the better ones.
Sapphire moon...Perhaps your thinking Cockatoos they live upwards of 20 years though its closer to 40 or 60...possibly 80...\
The bird should be taken to the vet a very common problem in birds are crop infections more common in babies but i mean if he/she isent acting wierd maybe you just have an over active bird? Try offering him treats like crackers and some fruits (BE CAREFUL SOME ARE TOXIC!) this should help with the underweight problem! Hope it helps