The question is broad and actually opinion based. Nevertheless, let me share what I practice.
We address people with Hello in emails quite commonly and this can go for every email you write or respond. However, if you are responding merely with a sentence or two, addressing person directly with their name (as John might have done in this case) is okay.
In another case, if it's the same conversation (more like a chat) having 2, 3 or more mails, I drop Hello and directly address the person with their name.
In this case, as I already said, you may choose any option - Hello John, or John. But if you want to sound a bit formal, go for Hello John,