Would the prob be in the turn signal?? His signal works but brake light doesnt. Im thinking that it is in the bulb socket, but have been wrong many times before. Get someone to step on the brakes, grab a test light and start messing with the socket.
I'm pretty sure the problem will be with the switch although the grounding of the socket should be verified by running a temporary wire from the socket to a bare metal part which is solidly attatched to the car body.
The brake and turn signal circuit use the same wire from the turn signal switch back to the contact in the bulb socket for each side of the car.The power for the brake lights goes through the turn signal switch after the brake pedal switch so the light will still flash when the brakes and turn signal are both on at the same time.
If the turn signal works,it means the wiring,the socket,and the contact in the socket are ok but there's no power for the brake light coming from the turn signal switch which usually means a bad contact inside the switch.
I just went through that with my 70 Roadrunner and ended up having to replace the switch which fixed the problem.