TB test positive

My son just took TB test and got positive result.The result of x-ray is normal.He took a BCG shot when he was in China in April 2003.He is 3 years old now.His doctor said he need take medicine for 9 months.I heard from others that it is normal that TB test result is positive if he took a BCG shot.I asked the doctor.But the doctor insist that the effect of BCG can only last for 1 year.After 1 year,if TB test result is still positive,that means he was exposured and he need a treatment.I know the medicine is very strong and my son is just 3 years old,I don't what will happen.So I haven't let him eat the medicine.How can I let the doctor change his mind?Any suggestion or expierence about this case?Thanks a lot!!