He and the dragon that sent him back him then work together to kill the dragon king or something like that. In the new timeline, the dragon that sent him back does not initially know him, but the hero gains her trust by showing her that he knows some things that there was no way he could have known. IIRC, the hero also manages to save a talented sword user girl that was his childhood friend or something in this new timeline. The enemy dragons all have ranks and there are some cool battles. The allied dragon that initially sent the hero back in time was undercover and other dragons do not know that she is/was working against them. Its was a shounen/seinen Japanese manga that was black and white.
This I'm less sure of but I think it was a monthly manga with 30-40 pages per chapter. May have read it on either mangadex or mangapark.
- A Returner's Magic Should Be Special