Thank you very much for your help ! :)
I am so sorry to have missed your question!
He may have iodine deficiency, and/or hypothyroid, which can predispose him to upper respiratory infections or, if he has a goiter due to an iodine deficiency it could create a narrowing in his respiratory system (trachea), which could account for increased squeaking. An upper respiratory problem would account for sneezing, heavy breathing, and warm feet, Hypothyroid could account for his being "chestier", weighing more, and seeming to lose his balance when grooming.
These are problems that may require medications, such as iodine supplementation, thyroid medication, and/or an antibiotic. Therefore, it would be best if you took him to an avian veterinarian for evaluation and possible treatment.