As far as I can remember, there is a bug which means that the script to make him speak to you when charmed never actually works. I did read somewhere how to make him speak, but I think he still goes hostile, but you do get the experience for not killing him (I think).
IIRC, he turns neutral for about 1 second, then green. I think you must attack him just before he goes neutral to get him to speak (but only attack him once). It's tricky but soemhow I managed to get it to work. As soon as I find the website I'll correct this if it turns out to be wrong.
Good luck
P.S- Found the website. Try this address
http://gamespot.com/gamespot/faqs/bg2.txt
You will have to search the page a lot because believe me when I say that it is BIG. Excellent guide though.
[This message has been edited by Mel (edited 08-18-2001).]