What to do if my son was injured by another vehicle and the driver did not have insurance but is now claiming it to be my sons fault?

Was there a police report?  If so, it will show who was at fault. 

If the other driver is at fault and your son has uninsured motorist coverage, your son can file an uninsured motorist claim with his insurance company.  If your son does not have uninsured motorist coverage, he will need to sue the other driver for negligence.  If your son is a minor, you will need to be appointed guardian ad litem to file a lawsuit on behalf of your son because a minor cannot file a lawsuit.

When your son completes his medical treatment and is released by the doctor or is declared by the doctor to be permanent and stationary which means having reached a point where no further improvement is anticipated, obtain his medical bills, medical reports and documentation of any wage loss.  If your son had uninsured motorist coverage, his personal injury claim consisting of these items should be filed with his insurance carrier.  Compensation for the medical bills is straight reimbursement.  Compensation for wage loss is straight reimbursement.  The medical reports will document the nature and extent of the injury and will be used to determine compensation for pain and suffering, an amount in addition to the medical bills.  If the case is settled with your son's insurance carrier, NO lawsuit is filed.  If your son is dissatisfied with settlement offers from the insurance carrier, reject the settlement offers and file a lawsuit for negligence against the other driver/registered owner of the vehicle if other than the driver.  If the case is NOT settled with the insurance carrier, your son's lawsuit must be filed prior to the expiration of the applicable statute of limitations or your son will lose his rights forever in the matter. 

If your son does not have uninsured motorist coverage on his auto insurance policy, be careful before incurring huge medical bills because your son will need to recover the medical bills, pain and suffering and wage loss in a lawsuit against the other driver, who probably doesn't have much in terms of assets since he doesn't have auto insurance.

If your son was at fault in the accident, your son's insurance company will provide him with an attorney at no cost if the other driver follows through on his threat to sue.