My son is 14 months and 19lbs what other tests could be done?

HE WAS BORN MAY 12, 2007 WEIGHING 6LBS 13OZ EVERYTHING WAS FINE, I FED HIM EXCLUSIVELY UNTIL 7 MONTHS THEN STARTED ADDING FOODS, KEPT FEEDING UNTIL A MONTH AGO WHEN HE TURNED 13 MONTHS.(HE SELF WEANED). IN THE FIRST 4 MONTHS HE GAINED WEIGHT VERY FAST. THEN HIS WEIGHT STARTED SLOWING DOWN. AND IT GOT EVEN WORSE WHEN I STARTED ADDED THE SUPPLEMENTAL FOODS. HIS DOCTOR WAS NOT WORRIED BUT SINCE HE STARTED GETTING CONSTIPATED SHE REFERRED HIM TO A PEDIATRIC GASTROENTEROLOGIST AND HE GOT CONCERNED ABOUT HIS WEIGHT.THEY RAN TESTS TO CHECK HIM FOR CYSTIC FIBROSIS AND IT CAME BACK NEGATIVE. TEST FOR DIABETED..SAME THERE SO THEY JUST TOLD HIM TO GIVE HIM LOTS OF FAT AND PEDIASURE. HE GAINED ALMOST A POUND IN 2 MONTHS SO WE WERE ALL HAPPY. THEN RECENTLY SINCE HE WAS BEING REGULAR I CUT DOWN THE MILK OF MAGNESIA BUT HE GOT VERY CONSTIPATED AND DIDNT WANT TO EAT. HE FINALLY WENT AND HE HAD A BIG ONE THE NEXT DAY HE GOT WEIGHED AGAIN AND LOST 8OZ. NOW THEY WANNA TEST HIM FOR MORE STUFF