Hyperthyroid could cause low weight and height, but you would normally see skin and hair problems.
I can't think of any reason to do the lead if he isn't developmentally impared.
If anything I would think he would be testing for possible food allergies, such as wheat and eggs, which can cause stunting..
But really if he has plenty of poop and pee, and it is normal in apperance you probably just have a small baby. Are you small? I am, at 5'1", I have small feet and...