[Charlie Chang]"How do I start tracking when his face starts to come into the shot?"
You can initiate tracking either forwards or backwards at any frame. I'd start near the end of the clip and track backwards and let the window go out of the frame... you may have to resort to the override mode and filter out tracking points that cause the shape to go non-linear as the control image goes out of the active raster. This will always be a shortcoming of point-trackers.
"I always pass on free advice -- its never of any use to me" Oscar Wilde.