Headlice treatment?

My 3yr old son has gotten headlice from his 3 older brother and sisters who come and visit every other weekend. When I try and pick through his head, he just cries and asks why am I hurting him. I've tried to explain to him why it has to happen, but he doesn't understand and still thinks I'm just trying to hurt him.

How can I treat his headlice without him thinking I'm intentionally trying to hurt him?