Your son is entitled to up to the limits of the person's liability if their bills and other damages are deemed to amount up to that much.

Your son is not automatically given the top limit of the person's bodily injury policy, $10,000 here for one person injured in an accident, because he was hurt in the accident. His medical bills, loss of wages, pain and suffering, etc are calculated and a settlement amount is offered.

While some calculated a settlement amount as medical bills times 3, this is not done across the board since different insurance companies have different guidelines and state laws can vary. 

If your son's medical bills are not for that much and his injuries are not severe then it is very unlikely that the settlement offer would be for an amount near the limit of the person's policy. If your son feels that the settlement is too low because he will have ongoing treatment, etc then he can negotiate for a higher settlement or perhaps hirer a lawyer to do so on his behalf.

For more information on your state's insurance laws concerning bodily injury settlements try contacting the consumer division of your state's insurance regulator.