There always has been a lot of speculation about Dabi being Endeavor's son. But yesterday I saw a My hero academia Youtuber who said in the last manga (#202) was strongly hinted so. What happened in the last manga? Is Dabi Endeavor's son?
There is still no confirmation, nor in my opinion enough information to jump to a definitive conclusion. However, considering the long running rumors and theories, I can think of what the YouTuber meant by a "strong hint".
I've tried to keep the spoilers for the current arc minimal in the following answer, so the actual plot of the arc has not been mentioned. It should be enough to ease your curiosity, but I can reveal more to you if you wish.
The information revealed in Chapter #202 was about:
Endeavor's eldest son, Touya. In a flashback from Shouto's perspective, we see Endeavor claim that Touya possessed firepower greater than his, but that he inherited his mother's frail constitution. He goes on to say that Touya was almost perfect, but not enough. And that Shouto would be the one to succeed his ambitions.
As to why this might be supportive of the theories that Dabi is connected to Endeavor:
We've seen Dabi as being capable of a lot of firepower, both during the Forest Training arc and at the end of the previous arc (Chapter #191). In addition, in the last panel of #191, Dabi remembers a hero named Snatch (whom he supposedly killed). Snatch yells at him about whether he had ever "stopped to think about how their families feel", to which Dabi comments "Haha.. I went crazy thinking about it". This could be referring to how he felt as part of Endeavor's family.
Chapter #290 of the series finally confirmed what many Dabi fans have thought all along. In front of both Enji and Shoto (Endeavor's younger son), Dabi revealed his true name, Toya Todoroki.