Though low levels of white blood cells indicate immuno compromised state & elevated ALT indicate a derangement in liver functions, in the absence of any specific signs & symptoms, these values cannot be considered significant.
The causes of constipation
1. Any pathology/ sub acute obstruction in the Gastrointestinal tract
2. Irritable bowel syndrome- constipation predominant type.
3. Functional cause- due to decreased gastric motility
4. Less fluid intake- as already mentioned in your post
5. Hormonal & Metabolic causes, etc.
Though symptomatic relief can be obtained by intake of Laxatives, their regular use is not advised.
And as long as the underlying cause is not treated, symptoms will keep recurring.
So, kindly consult a Gastroenterologist
& get a proper physical examination done.
Investigations like complete blood count
scan of abdomen, Colonoscopy
& contrast studies will be helpful to rule out any pathological cause for the symptoms.
There is no need to worry, your son will be fine.
Till then, maintain adequate hydration & proper nutrition status.
Wishing him a good health.