On episode I saw he mentioned Shouto full name like he knew him so well not someone who watched the tournament on tv. I think its hints he is Touya and how it sad he was in ancient but he assumed dead. And here is Dabi burn injuries scares and stuff. Its known mother have cold like quirk it would explain burn mutilate scars because Touya must had his mother's genetics that would make him sensitive enough be injured by fire.
This is not a new theory I heard even manga hints that he is Touya. If anyone knows where I can read manga for free let me know. I love watching anime and love reading manga comic novels.