How can I persuade DH when he is balking & arguing when I correct him using his walker, sitting down correctly, doing his PT homework?

Maybe see if you can get home therapy for him. Medicare will pay for it every so often. Will need a doctors order. Sometimes they listen better to someone else.

Be aware that the first thing to go with Dementia is short-term memory. He may not even remember what you have said from day to day, hour by hour, or minute by minute. Then its reasoning. You cannot reason with a person who has a Dementia. They lose the ability.

Whether your DH has dementia, Alzheimer's or a brain injury that affects his short term memory, it's all basically the same thing. You can't 'correct' him because it won't work. You have to allow him to be in charge of his own PT and recovery b/c the more you get on him, the more he'll argue and carry on. That's the nature of dementia, unfortunately, and it's a no win situation for YOU. You can only do the best you can do to help him, and then let the rest go. "Motivating" him may also be an exercise in futility. His brain is not working properly any longer, so the best thing you can do is learn everything you can about dementia in an effort to help him to the best of your ability.

I suggest you read this 33 page booklet ( a free download) which has THE best information ever about managing dementia and what to expect with an elder who's been diagnosed with it. How YOU respond to HIM can make all the difference.

Understanding the Dementia Experience, by Jennifer Ghent-Fuller 
https://www.smashwords.com/books/view/210580

She also has published a workbook entitled, “It Isn’t Common Sense: Interacting with People Who Have Memory Loss Due to Dementia.” For people to understand that reality and context as perceived and experienced by a person with dementia, is altered by the dementia; and, that their reality and context is continuously changing as the dementia progresses, requires learning and an attitude shift; it is not ‘common sense.’

Here is a list of useful tips from her e-book I found to be excellent:

The “Dont's”
· Do not reason and argue
· Do not demand that they reason or problem-solve
· Do not demand that they remember
· Do not demand that they get their facts straight
· Do not correct their ideas or scold them
· Do not reorient them
· Do not think that they are being uncooperative on purpose
· Do not think that they really do remember, but are pretending not to
· Do not use a “bossy” dictatorial attitude in care
· Do not act with impatience

The Do's
· Enter into their frame of reality, or their 'world'
· Be aware of their mood or state of mind
· Use few words and simple phrases
· OR use no words, just friendly gestures and simple motions
· Do everything slowly
· Approach from the front
· Wait for a slow response
· Constantly reassure them that everything is 'OK'
· Keep people with dementia comfortable 'in the moment' - every moment
· Maximize use of remaining abilities
· Limit TV or radio programs which they may feel are frighteningly real
· Maintain privacy
· Provide a safe physical environment

Language Needs
· Use short words
· Use clear and simple sentences
· Speak slowly and calmly
· Questions should ask for a “yes” or “no” answer
· Talk about one thing at a time
· Talk about concrete things; not abstract ideas
· Use common phrases
· Always say what you are doing
· If they repeat their question, repeat your answer as you did the first time · Give them a longer time to process information
· Wait patiently for a response
· Be accepting of inappropriate answers and nonsense words
· Speak softly, soothingly and gently

Care Needs
· Recognize that receiving personal care feels intrusive
· Reassure with your tone and manner
· Do one thing at a time
· Talk through the care “play-by- play”
· Be aware of your body language and use it to communicate relaxation and reassurance
· Be sincere
· Use a soft, soothing touch
· Be aware of the individual’s unique triggers
· Be aware that a person with dementia may not accurately judge whether a situation is threatening to them
· They may respond to fear, pain or anxiety by defending themselves with what we call “aggression”
· If they become distressed, stop immediately and allow them time to calm down – don’t try to restart the activity right away
You need to change your behavior to adapt to the dementia because the person with the disease cannot.

Wishing you the best of luck with all you have on your plate.

To be honest, the very WORST person to assist with PT or just about anything else is the husband, wife, parent or child. It just doesn't work.
I tried to get my bro to do his balance exercises which in rehab had, he acknowleged, helped him so very much. But no, no and no. Moreover it pushed us into our respective corners, and I recognized early on was actually HARMFUL. Not helpful.
For your own frustration try to understand that some of this may be stubborn behavior, but the more likely reason is the brain injury. You can't argue with illness and disease. It always wins.
And then to put this in the hands of the doc. Let him know you cannot assist hubby in doing exercise and ask for some home PT help if possible.
Sorry you are going through this.

Can you think of any rewards that could inspire him? Activities such as taking a drive through a lovely area, stopping to get a Dairy Queen (if it's within his diet) and if he can get in a car?

What about playing his favorite music to motivate and inspire him? Music can change a person's temporary attitude and willingness to participate.

I would also search TBI injuries, find support organizations and/or groups and contact them.

Years ago I remember a friend was helping her brother with "patterning", a TBI treatment then in use. A cousin's niece also worked at a TBI rehab place and had a great attitude toward working with challenged patients. These are also something to explore.

The short answer is, stop correcting him.
Let someone else guide him.

Loozing forward to responses. We're in the same boat with my husband. Best of luck to you