Normal range of Calcium in children is considered as 7.6 to 10.8 mg/dL.
As your son initially had constipation
and later developed green stools, vomiting
,abdominal pain; it is indicative of intestinal obstruction
If vomiting is green/dark brown/red colored, then it is risky.
Another option is gastroenteritis; but it needs clinical examination to confirm it.
I would recommend you to take second opinion
. Get USG scan done, stool examination done. Early diagnosis and treatment is main aim.