Assalaamua'laikum warahmatullah

I have just begun giving private tuition (maths/science) to a Christian lad at his home. Its sundays 8 - 12 a.m. He is quiet, polite and seems very obedient.

Now I would feel extremely guilty if I just fulfilled my duties without introducing Islam to him, even though it may be in a subtle way. After all doesn't the obligation of dawah rises for those "under one's command"?

I would greatly appreciate if someone could chart a concrete plan for me, how to break the topic of Islam to him, and then how to proceed step-by-step? I understand dawah is our INDIVIDUAL DUTY wherever possible, and it seems there is a great chance here. Just that I am new and need some guidance. In the past, I did "bulldozer dawah" which probably caused more harms than benefits.

Jazakallah khairan katseeran.