It means he or she is being corgel and you should say "hi back ... I'm so helpful :)
My old friend keeps on saying hi to me at school. What does this mean?
Hi ! Hello! Hey!
What do these words have in common? They're greetings. I know, I know, it's a pretty simplistic way of looking at things, but I tend not to overthink it if someone says HI to me. Say HI back because you want to be polite AND nice. It really is just that simple.
That's how humans greet each other.
I say "hi" to many people everyday that I've never met. It is meaningless, I would more-so say hi to someone I've met before. SMH.
Like hi
They're being polite.
Probably a verbal acknowlegement of his memories of having had a rewarding relationship with you.