How doI defend against a personal injury suit?

Thank you for submitting your question. In response to your inquiry, whether your son's homeowners insurance will cover you (should you be sued) will be determined by the language of his insurance policy. You may be considered an "other insured" or "additional insured" if the policy truly follows the dog regardless of location or caregiver.  While a homeowner’s policy covers the homeowner and usually an individual that gets injured on the property, it may also cover other individuals against personal injury actions.
However, I would also note that insurance contracts can sometimes contain language which is confusing to the policyholder. While you have stated that your homeowners policy will not cover the dog bite, and not knowing how you came to this conclusion, you may find it helpful to consult with your insurance company's claims department to confirm that coverage does not exist (assuming you came to this conclusion by reading the policy). Further, if, after reading your own policy, you believe that coverage may exist, if may be best to consult an attorney-- just because your insurance company denies a claim does not necessarily mean that the denial was properly made. An attorney would be able to assess whether coverage exists and, if appropriate, seek the court's assistance in obtaining coverage for the dog bite claim.

Lastly, you may have coverage for the dog bite if you also took out an "umbrella", or personal liability policy. There types of policies generally cover nondescript events which cause damage and/or injury but do not fall within the coverage of any other policy.