Father of 8 yrs old son need advice on how to get child custody?

already divorced on mutual but mother has taken the childs. custody.... she is working now in other city and had left the kid with her parents. Hence father wants to take custody of the child. Father of the child is married and second wife is ready to take care of this child. Child is 8yrs and he is a weak child and physical development is very slow.. he has a gastroenterological problem and also advised by dr that he is a hyper active child. Due to his medical issues he needs a parents support to hv a normal growth. Father is working abroad and he is capable of taking care.