chest may sometimes show any residual disease in the lung in case of Lung TB.
When compared to skin test,Blood test
like PCR(Polymerase chain reaction) can sometimes be helpful in diagnosing previous TB.It may show False positive or False negative results as the person has taken treatment.
But,one thing has to be clarified is that Whether he stopped the treatment after Doctor's advice only or not.He has to go for follow up regularly.
Wish him good health