My Son Was Taken To The Doctor For A Cold,

My son was taken to the doctor for a cold, I told them about his frequent urinating.... I noticed it about 1 ago, at night he is going a lot.. but doesn't seem to be a lot of urine at one time... He is 12 about to be 13... He has always slept through the night, even as a toddler when he was potty trained, never wet the bed. The doctor had him pee in a cup and it was tested.... no infection was detected, there was protein in his urine and it was concentrated. This has not been the first time I have been told he has had protein in his urine... what should I do