Guy I don't know messages me on Facebook and wishes me happy birthday every year. Is it rude not to reply to his messages?

Anonymous

I don't know this guy. He went to my school many years ago I think when I was 13, but I've never spoken to him before in my life. I have him on Facebook like I have a load of distant acquaintances added that I never talk to. But every single year he remembers my birthday on my timeline and writes me a long message, which is nice I guess, but he recently started Facebook dming me asking if I want to do some work with him.

I thought it was a little weird honestly as we've never spoken, so I haven't replied. But now he keeps sending follow-up messages highlighting how I haven't replied to him. Should I reply and say "thank you but I don't need any work right now."? Have I been rude by not responding? I don't want to encourage the messages etc.

Updates
1 y
He even writes me belated birthday posts, and it's been every single year.
Guy I don't know messages me on Facebook and wishes me happy birthday every year. Is it rude not to reply to his messages?
2 Opinion