Hi, twiggy --
Would it be possible to get him to walk outside rather than indoors? He'd probably get more exercise and the fresh air has a good effect on most of us. If the two of you established a habit of a daily walk, I'm sure he'd benefit. If he is resistant to the idea, of course, I realise that you may not be able to do this.
I have not experienced that restlessness myself, but I have heard of other pwp who have. It's usually the result of difficulties sleeping, I think. Maybe your husband's PD nurse could recommend something for sleep.
I really believe you caregivers suffer at least as many hardships as we patients with PD. It is terribly hard to watch someone you love grow ill and feel helpless about the situation. I wish you well in your efforts!