Blood from toddlers penis

This has been ongoing for 8months now, started when he was 13m. Has anyone ever been through this with their toddler?? There's a blood blob, or a dark pus coloured blob in his nappy at least once a month. We've done urine samples - no infection indicated. We've been for ultrasound - no abnormalities and/or blockages in tubes. Back to square 1 with doctors. No-one has any idea what it is, it's like he's first baby in world to have this happen to them! Surely someone's heard of this before? It's been such a long time to go without a conclusion. Doctors say that it's not really a concern because he's absolutely fine within himself and no visible symptoms. But I'm concerned as its just not normal?! Was just wondering if anyone's experienced it before?