Minor child custody

Sir, My brother in laws marriage is in troubled waters. Wife is just 21 yrs old. They have a 2yr old son. As wife of my BIL is just around 20, their son was totally raised by grandparents of the minor child. Now when their marriage is in trouble, wife left home and took child with her. Sometimes when child is brought back, he does want to go back with her mother when she comes to take him. As his dadi was everything for him, the child. It is quite clear that the wife of my BIL will not be able to raise the child in a wholesome manner, as she is just 21, and no sourse of income. My BIL also does not have proper source of income. Only his parents are the one who has pension and rental income. Can this custody battle be won. As his dadi and dada only can give the child best upbringing. Plz reply.