My Son , He Is 18 Years Old And He

My son , he is 18 years old and he saw very small white articles in his urine (3 times in last three) years . It happened once and the same time he sees this articles he has burning for a half hour . I got him to his doctor and he did urine test and urine culture. In the urine test they saw modrate bacteria but the culture was negative. The doctor prescribed him antibiotic but since the culture was negative I haven't started the medication. What do you think?