Legal heir

Adoption can be challanged, however, it depends upon the detail facts and circumstances of the case.

Even if courts hold that there is a valid adoption, in the immovable and movable properties of your uncle, his mother and wife have got legal share.  So, if adoption is valid, uncle's mother and uncle's wife and that boy get 1/3rd share each.  If adoption is not valid, uncle's mother and uncle's wife get 1/2 share each and that boy do not get any thing.  If the boy does not move the court to prove adoption and keep silent enjoying the property, then there will be survival problem for these two ladies.  So, they themselves have to move the court for partition of the property.  Whether that boy should be made party or not to be decided by knowing full facts and their affection towards that boy.

As he has no legal adoption prove except educational certificate how we can challenge against him and what are the facts we require to win......

If he is claiming himself to be an adopted son of the uncle, let him rove it, mere name in the educational certificate will not entitle him to be an adopted son of your uncle. As far as the property left behind by  the uncle, his legal heirs namely, his mother and wife are the only successors to the property, let him come through court seeking partition, his suit can be challenged properly.  Let the court decide about his heirship to your deceased uncle.

Fact of adoption cannot be challenged because no person would dare to suddenly lie before the court on oath that he had not been grown up at home.  Adoption need not depend strictly on documentary evidence. However as Adv. Chandu had said, if the two women are content with 1/3rd share each, they can file a partition suit.