If it's in rhythm with his heart beat, it may be a damaged heart valve. He needs to be seen by a doctor. If it's in rhythm with his breathing, it could be some mucous rattling in the lungs. Again, he needs to see a doctor.
Im wondering that this clicking noise could be a muscle or tendon popping everytime he inhales or exhales.
If you can actually HEAR something without the aid of a stethoscope, I'd go ASAP to either your doctor, or a hospital. Don't wait on this!