I don’t know exactly what was said in the request, but it’s possible that he grew up since your school days, considering how empathy is a thing kids learn and aren’t just gifted with. That said, I can absolutely see why it was a massive shock to you to get that friend request so suddenly, and I’m so sorry to hear about it.

Book a therapy session to deal with your bullying issues. Then don't pay for it. You'll feel a lot better for it and he won't want the bad press of trying to sue you for it.

I ran into my high school bully about 10 years ago, after not seeing him for over 15 years. He apologized for his treatment towards me in high school and he and I are good friends today. People grow with age, he may have matured and sent you the request to try and make amends for how he treated you years ago.

Accept his request and get free therapy sessions. He'll gaslight your into believing his "bullying" was him defending himself from your bullying. Cheers!