Homeowner insurance pays for injuries caused by personal actions of the insured. Most losses involving an automobile are excluded, since it would be covered by an automobile policy.
Since your husband was using a tool he had at home and a piece flew up and struck your son, the insurance company should pay for the injury to your son.
The homeowner policy has two areas that should allow some payment to your son. The medical payments portion can pay for injuries to non-residents whether there is any negligence by the insured or not. This amount could be between $1,000 to $10,000. The other portion would be the liability coverage under the homeowner. In this case negligence is generally required before the insurance company will pay.
You should contact the adjuster for your insurance company and press them to review the claim. If they decline, you should ask for a supervisor and try to get them to reconsider. Explain that the injury was caused by a tool being used at the home. Again, the fact that the repair was involving an auto really should not be a part of the decision.