Wow! How scared you must be! Any time you don't have weight gain, it is a concern. It is called failure to thrive. It could be from malabsorption because of several causes. It could be that you and your husband have small genes or carry small genes and your son is growing fine according to his genetic makeup. Or,it could be Cystic Fibrosis (CF.)
CF is basically a chronic disease where all the body's secretions are really thick. It affects the whole body because there's a lot of vital areas of the body where if there is thick secretions, it could cause a problem: lungs, testicles, and bowel. Those are the major areas hit by CF. The sweat test will determine the CF diagnosis. The patient is basically sterile (unable to reproduce) and has to be on a special diet to keep the bowel healthy and a vibrating machine daily to keep from getting pneumonia and other complications.
I hope you will be able to get some answers soon. I wish you the best! I hope you have had your questions answered here too. If not, let me know!
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