Any recommendations for antidepression rx that works well on an Alzheimer's man in his 70's?

The best antidepressant for you husband is the one that works well for him. Alas, that is pretty hard to determine until he takes something. A geriatrician or a psychiatrist may be most likely to evaluate the situation accurately with the fewest trials, but any doctor may need more than one attempt at finding the best drug for a given individual.

What kind of doctor is treating your husband's dementia? Have you discussed this with him or her?

There is no one size fits all medication. All patients respond differently, and interactions with other meds are a big consideration. Very often Alzheimer's patients react badly to SSRI meds commonly used in depression. Talk to your pharmacist, they know a LOT about new meds.