Ok, so I decided to message him, and here's what I said:
"Hello XXXXX, remember me from school?
I was the one you used to bully. In fact, you and your friends bullied me so much that I ended up developing severe clinical depression and social anxiety disorder. By the time I left school I was so ill that instead of going to university like everyone else, I saw a psychiatrist for two years and a clinical psychologist for 6 years.
When I was 21, I was diagnosed with Asperger's Syndrome, which is a "mild" form of autism. It means I have difficulty understanding social interaction, so I tend to be quieter than most people, and other things like poor co-ordination, making me bad at sports, etc. I guess this is what made me so different, that seemed to make people like you want to treat me the way you did.
I realise it was a long time ago, and I'm not really sure why I wanted to contact you about this now. I don't even know if you remember, or if you were aware how much your behaviour affected me. But actions have consequences, and I felt the consequences of your actions for a long time.
Anyway, I'm not writing to you for sympathy, a lot of people have worse problems than I do. I guess I noticed your facebook page and saw you are a Christian. I suppose I wondered if you really had changed.
I see you are doing pretty well in life. I am glad for you, and congratulations on getting married. I hope that if you and your wife ever have children, that none of them are ever treated the way you treated me.
Just had to get that off my chest."