They say that individuals who use a knife, and stab in a frenzy, are prone to wounding themselves because the knife gets slick with blood, and your grip slips. He could have dripped blood from his own wound. He could have left sweat, spit, hair, or anything at the scene that could have been tested. Don’t know if he was wearing disposable gloves. Maybe the dummy took them off at the scene, and left them. People aren’t in prison because they’re smart, and DNA is pretty advanced these days. It’s also possible that they have him on closed circuit TV. From what I’ve read, the police have been pretty tight-lipped about everything, which is the way an investigation should be conducted.
His blood could have left drops outside—certainly a good place for forensic serologists to start their analyzing regimens.