Возможно, ему не удалось доказать,

Возможно, ему не удалось доказать, что он прав.

Результаты (английский) 1:

Perhaps he had failed to prove that he's right.

Результаты (английский) 2:

Perhaps he could not prove that he is right.

Результаты (английский) 3:

It is possible, it has not been able to prove that he was right.