My son broke his wrist near the radial artery, now it pops when he lifts his arm and has numbness in his thumb. Is this normal? The doctor splinted it until the swelling goes down to have a cast put on.

Nerve injury. It seems that with fracture of wrist he also injured radial nerve which supplies sensations of thumb on back side.
Its . Its not a typical sequelae of a fracture of the wrist, however injury to the radial sensory nerve can give numbness on the dorsal aspect ( the top) of the thumb. Injury to the median nerve can give numbness at the tip of the thumb ( as well as the tip of the index and middle fingers) popping can be instability of the joint where the radius and ulna meet. Or it can be a problem with the main joint of the distal radius. Severe injuries can often give results that are associated with these problems. So rather than say it is or is not normal, he should be evaluated. If it is an acute injury it is time to see an orthopedic surgeon or a hand surgeon who deals with complex wrist fractures finally the numbness may be due to the splint or wrap being to tight or swelling from the fracture. Another good reason to see someone. If you cannot get into see someone soon you should contact the doctor who put the splint on to make sure it is not too tight or that something more needs to be done.