At one point, they were saying how Hiro was being mutated by Zero Two, that he soon wouldn't be considered human. I think you're right about that.
I was just going to make a post asking about your question however~ in the beginning he was getting some kind of growth on his chest..which also left a scar, and it was killing him until episode 6 when it all suddenly disappeared. I didn't understand how that made it go away, or if it did go away..because I think that blue growth was part of the mutation.
The reason he survived is because I think he fully mutated into the same claxo-human hybrid that zero two is. That would explain why he didn’t start to grow the horns until after that, and why he is now able to survive a massive amount of magma energy (yellow blood cells) in his blood stream