Legal Question in Personal Injury in California

I assume that this was a game at one particular high school field and not a league called Pioneer. Is the injured boy in a 4 year level high school as opposed to three years? If not, why was he on the high school field if he still was in junior high?

You fail to describe how the incident occurred. Many injuries on the athletic field happen with no one being at fault. That the other person was 18 does not necessarily mean much; was he much taller, heavier, stronger then the other players? Would shin guards have prevented the injury? Was there or should have there been any direct supervision by a coach?

The general rule is that people playing in an athletic contest assume the risk of suffering injuries from actions that would normally occur during the activity. Otherwise, there would be too many lawsuits. But you provide insufficient facts for any attorney to reach a conclusion. There are special rules for schools, but the 18 year old could be sued. For all governmental entities, any claim against them must be filed within 180 days of the date of the injury [motion for relief from claim requirement filed ASAP but no longer than one year

Part of the recovery that would be allowed if there was negligence is pain and suffering.

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Unless there are witnesses who will testify that the other player intentionally went to break your son's leg, I do not think that there is any cause of action. Your son played a sport and became injured. It happens, that's life. I wish your son the best recovery.