If your talking about the topside of the forearm, as when the palm is facing the ground, then it is most likely not the ulnar nerve. Look up radial nerve entrapment and also supinator syndrome to see which symptoms most closely match those of your son. Both are nerve issues (affecting different nerves) and can be difficult to manage in a thrower. They can pop up without much warning or trauma and can be very tender to pressure that is applied directly to area affected.