Why my child has yellow eyes and itchy body and how long this will take him to get back to normal?

While it's impossible to accurately diagnose a malady with so little information, these two symptoms == yellow jaundice and systemic itching -- can occur when the body builds up an overabundance of bilirubin, a bile salt and pigment. This is a symptomatic indicator of reduced liver function. The extent of liver damage, whether it's permanent or temporary, &c. all require more data to determine.

As a sharp an increase in bilirubin can be the symptom of very grave damage to the liver, I strongly advise that you get this problem in front of a doctor immediately. Not tomorrow -- now. Good luck.