Husband has dementia and Parkinsonism. His neurologist had him sign up for physical therapy. He listens and tries only while there. Advice?

It is not wrong for you not to want to coach your husband, especially as things are with its not working for either of you; but it would be wrong just to abandon the practice sessions.

Options:

get trained in how to lead these exercises - ask the PTs who work with your husband's neurologist to show you, and to let you participate in outpatient sessions. Your husband's dementia perhaps means that he needs the instructions you give him to be very brief and very simple, e.g. "right foot. Left foot. Hold here (guiding his hand)." Again, this is something the PTs should be able to explain.

or

hire a home health aide with experience and/or training in supporting PT to come to your home.

Truthfully, I believe physical therapy will help the dr more than a patient w dementia. They simply will not cooperate w instructions or anyone telling them to do anything. They just want to be left alone & have nobody bother them. I can’t tell you how many times I tried to have my mother get physical &/or occupational therapy. She just refuses & tells therapist to do it or sends them away. Hugs 🤗

Try movement therapy.  Put on a CD of his favorite music; it can skip the PT instructions and move directly to stimulate his brain (unless he hates music).   Start moving, swaying, even tapping your feet, just to get him started.  

You might also want to read up on the use of music and movement as therapy for Parkinson's Disease patients:

https://danceforparkinsons.org/

I have fought the PT battle with my 92 y/o mother for years now. Despite the fact that she knew she'd become wheelchair bound, she STILL refused to do the 'homework' she needed to do to remain ambulatory. We went back & forth for years, and she's now wheelchair bound, due to her refusal to put in the required effort to stay OUT of the wheelchair. Her choice.

You can't force your husband to do something he doesn't want to do, or something he doesn't understand. I guess you'll need to let it go and not think you're doing something 'wrong'. You're doing the best you can.

If you can get some in home PT and ask them to teach you in the environment that you will be helping him would make it easier for you.

My dad can understand others far better then he can me. I think it is a game that he plays to not have to do what I say. Which is only what the doctor said, but he can't swallow it coming from me.

I never had any luck with exercises, I just took him places that he had to walk. When he got tired I would take him home. I figured at least he was moving so it had to be beneficial.

I would call his doctor and ask if they could prescribe some in home PT and maybe some OT to help ensure that the house is set up to be the easiest environment for you to help him.

Boy can I relate. My husband has the same diagnosis with orthostatic hypertension ( low BP ) to boot. He’s had physical therapy two different times, but he doesn’t continue the exercises. I try to encourage him, but he’s either ‘frozen’ or sleepy, or light headed. It’s ridiculous!
We would walk as much as we could, while he could.
I suggest walking together; and don’t be hard on yourself.
These diseases are hard enough.

The Parkinson's/Dementia combo is hard to overcome. My mom couldn't get past the immediate discomfort of PT to see the long-term benefit and just refused to work with the therapist. She became bed bound within three years of her diagnosis simply because she refused to move. Now I can barely get her to lift her hand to scratch her nose. It is frustrating, but there is absolutely no reasoning with dementia. My mom barely comprehends the concept of "tomorrow" and certainly not "the future".

I gave up trying to get my mom (with dementia) to do anything physical.  She gets mad and says she walks all the time.  The only place she walks to is the dining room to get her meals.  It is useless to have the conversation.  She doesn't remember it.  I have accepted her limitations and I am trying to enjoy what time she has left.  Acceptance of her situation was huge for me.  It took a lot of the stress away that I was carrying unnecessarily.   

I hope you get there.

I am in the same situation. My husband, with Parkinson's and Neuropathy  is in a wheelchair and refuses to do any exercises what so ever !  There are 6 steps he has to take to get to ore car which is in the basement of our building. When he wants to go out ( nearby casino) that is the only time he will venture to "move" .He makes the steps going down, but very difficult going up.. But he does it ? So I consider that therapy. Ha Ha. What can I say ? Sometimes they need an incentive (like a child) to get them going. I told him I would keep a chart and give him stars each time he attempts some movement (like a child) and it works at times. Humor helps also. Don't give up but do the best you can. I know, "it's tough" !!

Discuss this with the PT as well as the dr. If there has not been an improvement then I would see no reason to continue. And the PT's notes should reflect that.
If this is frustrating for him and you that is another reason to discontinue PT.
Take what you have learned and try to adapt it for what you both do day to day.