With in my own limited knowledge, I find it Virtually (no pun intended) impossible for him to find out what new accounts you make. Unless of course, he has a trojan keylogger in your system.
A trojan keylogger, logs all keyboard strokes, and can also mention what program or application they were captured from.
This will entail, your SL chats, your SL login names (which are your new account names) and SL password to be more precise, which is more dangerous.
You should also try to see, he may have logged in from your accounts as well.
Anyways, with a keylogger he can keep trace...but thats the only way I can think of....
Best bet? format, reinstall OS, re-install SL, if you have dynamic ip, re aquire ip, change e-mail....maybe he'll get off your tail then..but all that is too much to ask now, isnt it?