Thanks for query,
First of all he should undergo urine test to confirm weather blood cells are there in urine? is there any presence of pus cells along with endotheial tissue.
The first possibility is of infection as he has dental infection may cause infection of urinary tract
The second possibility is of any bladder or urethral polyp.
The last possibility is of a color changed of urine as some antibiotics excreated in urine may changed the color of urine.