Yes, a power of attorney can be rescinded and changed, but the change must come from the principal -- in this case, the wife and mother of the two souls
So the husband's best route may be to get a conservatorship, sometimes called an adult guardianship, giving him the legal right to make necessary decisions on her behalf. Laws in every state provide specific criteria for evaluating whether a person qualifies for a guardianship or conservatorship -- and this legal arrangement requires some paperwork and a court hearing.
Generally, the husband will have to explain to the court why he is the best person for the job. And he should be prepared for the possibility that the son will fight him for it. It will then be up to the judge to decide who is best fitted for the task.
Whether the husband will need to hire an attorney for help with this process depends on where he lives. Courts in some places have established some very good self-help centers that provide necessary forms and instructions for how to complete and file them. Find out by doing a search of the city or county and the words "conservatorship" or "guardianship."
If the court does not provide good help, or the idea of going it alone just seems too overwhelming, he should be able to get referrals for experienced legal help through the local or state bar association.
who are disagreeing over her care. And it sounds as if her advanced Alzheimer's may prevent her from having the legal capacity to make this change.