Sounds like you need to be referred to a specialist childrens hospital in endocrine, assuming he isn't already under them. You can demand on wards referral if you feel you're not getting anywhere (you most likely know that though).
A girl at my hairdressers her son was born with a very strange thing cant remember what its called, but he had some scary high and low blood readings, they struggled with his feeds etc, he's about the same age as your son and he was referred to Royal Manchester childrens hospital, as they have specialised endocrinologists and this thing he had they said he would grow out of and he did. i couldn't believe when she said, i said still better to keep an eye out in case it came back when he was older. He did have some other health issues, but not majorly serious and was a month premature.
How did you spot that something was amiss?