When is it safe for a child to stop using a booster seat in car? He is 8yrs old 77pounds

It is safe. Laws vary by state. I believe in Texas, children over the age of 8 can ride legally without a booster. In California the current laws is: ?Children under the age of 8 must be secured in a car seat or booster seat in the back seat. Children who are 8 years of age OR have reached 57" in height, may ride in the back with a seat belt.
4 foot 7 inches. The seat belt is designed to transfer the forces of a crash to the adult pelvis, which it will miss in any kid under 4'7".Kids prematurely moved to a belt are at risk for substantial abdominal and spinal cord injuries. Texas law was a balance between forces who wanted no law and those pointing out the need.It is not the recommendation, it is how to avoid a ticket.