The average RESTING pulse for a child 7 to 12 years of age is in the range of 75 to 110 beats per minute. I agree with Michelle. Your child's pulse seems a bit slow.
When was the reading taken? At rest or after he was active? Have you tested to see how his heart responds after he has finished exerting himself? The rate seems a bit low especially for a child but you say he trains. What type of training? Normal heart rate for adults is 60-100. Not sure at what age the child's rate is considered comparable to an adults. Does your child complain of any symptoms? You should probably get him checked out by a doctor just to be sure it is safe for him to exercise. He could have something as simple as a block but better to let a doctor look at him. Take care.