My husband is trying to add my son (his stepson) to his health insurance plan?

I already have my son on my policy from my job. My husband was trying to add my son to his family plan at the same time he added me right after we were married, but my husbands HR director wouldn't let him. Is that common? Our lawyer did go through the courts to have my husband named as a guardian of my son. Is there any reason he would not be able to have him on his family health plan as well? I am just confused
--------------------
You do not need to adopt to obtain coverage, legal guardianship is all that is needed. You should call the insurance carrier directly and explain to them what his employer said. If you tell the insurance company that other coverage will not be dropped they might not issue new coverage. Having double coverage is usually not worth the price you pay.
Source