What are the recommendations for using a booster seat?

Not quite!  He should remain in a car seat until he is officially four years old.  The reason for the combined milestones of four years and forty pounds is that size and development are both important in terms of neck strength and head control. 

Beyond four years and forty pounds, a booster seat with a five point harness that can convert into use with a seat belt with shoulder belt is strongly recommended.  Boosters are recommended until a child is four feet nine (57 inches) and while the older children in this group do fine with the lap and shoulder belts, the four through at least six year olds need the protection of the five point harness. By 57 inches, most children can be safely protected by a lap and shoulder belt, in the back seat.