I think if you just want to be his friend, then you shouldn`t use the term "勾引".
My suggestion is, if you have any other male friends, maybe you can treat him as one of your male friends, but that`s just it. You can`t get any closer because he has a girl friend already.
Seducing won`t work if you want to be his friends, and there may be great chance that it will be out of control. It won`t be a challenge that you can take if you are really as shy as you described.
Just keep steady and show your sincerity.