Does Rex have pit bull in him??

2 rows of teeth are the baby teeth and the adult teeth, just the baby ones didn't fall out. The only way to know his breed history is possibly through a dna test, but even then the breed has to be 10% present in a dog to pick it up in the test. He could be 75% lab and 5% of 5 other breeds and the test would just show the lab part. If he is aggressive at 15 weeks, he needs some major training and socialization before he becomes uncontrollable. You should also neuter him as soon as possible.