The answer is "a little bit of both". Title to the assets in a POD account pass automatically to the POD beneficiary, regardless of what the will says. As a result, the POD designation to your husband will trump the Will in almost all cases. That being said, if there are inadequate assets in the estate to otherwise pay the decedent's expenses, creditors and taxes, then the POD assets are subject to claim by creditors, and may be independently liable for inheritance taxes in your state.
Please recall that this is general advice of a public nature, and we do not have an attorney-client relationship. Mark T. Coulter is admitted to practice law in the Commonwealth of Pennsylvania.
Contrary to what you may have been told, a POD designation may indeed be changed through "specific reference by will". The end result will depend upon the facts of your situation, but as a general proposition, there is clearly a legal obstacle for your husband to surmount.