Her DPOA says she does not have dementia. However, he will not allow her to go to her home and go through her belongings to see what she would like to have with her. I asked if this was temporary, but he said, "no" she will never go back to her home. Is this legal? He doesn't even want me to visit her alone because he doesn't want her to know. What is best?
It is really hard to know what is best for your mother without a whole lot more information. In many cases it would be best for the person not to return home. In others it might be appropriate to go, say goodbye, select some items, etc.
It is also hard to know what is "legal" without knowing exactly what documents are in place and your mother's state of impairment.
But I suspect that what is best for your mother includes lots of visits from you. If you promise not to bring up her home, would the DPOA be more relaxed about you visiting? (He may not have the legal authority to prevent you, but do you really want to get into a legal fight with him?)
Focus on what is best for Mother at this point. You may learn more about that by visiting often.