As i known,gain one`s informations via yahoo mesenger is nearly impossible(except Contact Deatils but this seems useless

)
So if i were you,i would do some tricks to cheat him,make yourself a website that force him to type his information,and all of that will be stored in your database.
One another way is to find a website that he usually visit and you know that his infos are there,then use some warez that can extract data via $_POST method.
I havent tried anything above,but i hope it can help,thats all i know.