Mr. Ballard is not entirely correct. The often-stated "6-month rule" is a myth. Jurisdiction is something a court can take under the Uniform Child Custody Jurisdiction Enforcement Act (UCCJEA) and you need to be prepared to effectively object to the California proceedings, at least as to the custody allegations. The UCCJEA will require the Idaho judge to contact the California judge to attempt resolution of the jurisdiction issue. Your Idaho divorce is not going to be a DIY operation as your pleadings need to reference the California action, of which you have knowledge. You will definitely want to be represented by a family law practitioner well experienced in the UCCJEA.
Best wishes for an outcome you can accept, and please remember to designate a best answer.
This answer is offered as a public service for general information only and may not be relied upon as legal advice.
Based on the facts as you have presented them, the court in California would not have jurisdiction to hear the case. An initial custody determination must be held where the child has lived with at least one parent for the past six months. That doesn't prevent your husband from trying to file in California, though. A competent family law attorney will best be able to protect your interests.
Ryan Ballard is licensed to practice law in Idaho. He can be reached by phone at 208-359-5532 or by email at firstname.lastname@example.org. His website is BallardLawIdaho.com. This answer is for informational purposes only. It is not intended as specific legal advice regarding your question. The answer could be different if all of the facts were known. This answer does not establish an attorney-client relationship.