To avoid confusion in this thread.
Bat speed is the speed of the bat itself. (This is problematic since you don't know where in the swing plane you achieved the speed - ex after contact, etc.)
Exit speed is the speed of the ball after it hits the bat. (Tee is best for consistency, or use same pitch speed, and bat weight/type)
Exit speed is a great indicator for scouting (if same pitch speed and bat type/weight). Great MLB (wood bat, and 90 mph pitches) exit speeds are 100+. Stanton hit 119.9 this year.
What speed should he be targeting?
Just get faster each time. Try different things and see if they are faster.