My 13 year old son has pain in his throwing elbow (inside of arm) He hasn't played ball in 6 months but this?

past weekend when pitching to his dad, the pain flared up again. I'm bringing him to the Dr. today but I've been told not to let the Dr. just tell me to "rest it" because he rested it for 6 months and as soon as he threw again, it started to hurt. His dad is concerned it's serious but I don't know how far to push his HMO Dr. for more treatment.