Blood in urine is pain less many times and may not be visible but he may be having it as it is diluted by urine so do see a doctor whrere his blood and urine may be examined to rule out any significant problems like any Infection , kidney
stone , or a kidney pathology like Nephritic syndrome
where we need to check Blood Pressure also even at the age of 12 years . All things may turn out negatve then we will be sure that we have not missed any thing . All the Best.