How can I get transportation to and from medical appointments for my husband who is in a power chair?

Call your local city government and ask if they offer transportation for disabled seniors. Our does. The fee is minimal, like a few dollars for one-way transit.

In our state we have transportation for seniors/lower income people that you can apply for through the Dept of Aging where you live. You must fill out an application and get approval. Then you pay a nominal fee according to his income. In my area it is called "MetroAccess". They provide transportation to doctor appointments and are equipped with wheelchair lifts. For instance, many wheelchair bound dialysis patients use this service to and from dialysis.
I'd look up Dept of Aging/Senior transportation in your county. I don't think they will take him everywhere, just to and from medical appointments.

He uses a hemiwalker to walk very few steps when absolutely necessary.

Some areas have taxi service vans where the back door of the van opens up and there is a ramp where one can ride up in a power chair. These are a bit pricey, but much cheaper then hiring a patient transfer van [which looks like an ambulance].

Can hubby use a rolling walker?