I'm 100% sure he is one.
I mean they eyes and power match
He has a grudge against Endeavour
He only adressed Shoto during Bakugou's kidnapping
And he ressembles them.
Plus there was a theory that the author likes wordplays and that Dabi's name could be Haruto with it meaning first son and first spring if I remember and how eache of the kids have a years name. Natsu=summer and so on.
And I think somewhere in the kanji of haruto there was something with Endeavour in it
（/TДT)/
I believe he was trying to remember who "Snatch" was, as Endeavor mentioned he murdered him. On the last page he remembered and said "Oh yeah, that guy. I thought so hard on it I was going crazy."
It shows that he murders without care and doesn't even remember who he harms.
Yo! That moment someone was talking about family and Dabi said
“ i thought so hard in it I went crazy. “
I’m not fully sure what it means but going with the theory that hes a Todoroki. Was he thinking about his mom??? Reí?