Do I Have to Take Secondary Health Insurance?

My son is 23 and just got a job that offers health insurance. He is still covered under my husbands health insurance plan and has better less expensive coverage with my husband than the plan being offered at his new job.

Does he have to take the insurance plan at his new job?

If he does take the insurance at his new job, can my husbands plan become a a secondary plan to his new plan and thus pick up costs that were not covered?