"You are right" is correct.
In context, you are not trying to say that the PERSON is true, but that what he SAID is true. You could say, "That statement is true" or "What you said is true", etc. But not "You are true."
There is a slightly archaic expression "you are true" meaning "you are living up to your commitment". This is most commonly used in reference to marriage vows. Like, "Oh darling, if you marry me, I will always be true to you" meaning "I will never have an affair with someone else". It is also used for other commitments, like "The senator rejected the bribe and remained true to his oath of office", or in general "He stayed true to his promise."