re-reading chapter 1, i can't see where he did. Only saw both of them pretty bloodied after the truck hit.
It was the anime that gave me the idea, in that you can clearly see her give him some of her blood.
It was also shown during the sequence where he recovers the rest of his memories from their first meeting, too.
Ch.147, Pg.8.
However... you're making a huge leap of logic that isn't supported by the rest of the manga: It has been shown a number of times since his initial encounter with Tsukasa that he can still get sick and still suffer normal injuries that require bandages and such and take time to recover from. These are things that contradict what Tsukasa herself has said are part of her abilities a couple of times: that she quickly recovers from injuries and cannot get sick.
If he became immortal back then, he would have had the same huge ass all consuming fire thing going on that Tsukasa did, too, in the moment of consuming her blood (Tsukasa's origins flashback arc:
Fly Me To The Moon: a life's final embers Ch.2, Pg.9 and
Pg.10). Consuming the blood isn't sufficient. The ancient incarnation of Kaguya explicitly made the Hourai Elixir from her blood, but it isn't just the blood of an immortal- something else unexplained was done to the blood so it could create a new immortal.
The blood on its own is just a temporary healing salve. She gave him a reprieve for awhile before he'd still need medical attention lest he actually die and stay dead. It's why it was important for him to stay behind and be found and rescued by the paramedics and police that were arriving that night while Tsukasa ran off.
I thought the kid was a girl too
The translation notes were pointing out that Mao was a historical figure known as
Kukai. Also, it seems a comment for the prior chapter pointed out that Mao/Kukai was misgendered with the initial upload, and that appears to have been fixed since then. You might have seen things before the fix was applied for the prior chapter.