Ditto the other posters.
But that being said, generally speaking, Tennessee Walkers are known to be very gentle and forgiving of human error --- generally speaking.
I have three TWH's. Two are like that, the third one is A.D.D. And would take control over anyone he can, given half the chance
In his defense he has oat/corn/soy allergies and when March gets here, he battles dust/mold/pollen allergies. He can't have more than one or two cube treats of timothy/alfalfa every day or that handsome-drags-the-ground-tail of his starts flicking and I have to wallop him.
When Tennessee Walkers get ready to move into their intermediate gait, it feels like they are trying to run away to a novice gaited horse owner.
Their hind legs with come up underneath themselves in a tremendous propulsion of motion (I call it turbo boost) and oftentimes riders new to gaited horses will pull the horse back because they think it's trying to run away with them and it isn't. That, in turn, sends mixed signals to the horse and he doesn't know what to do.
It might be a good idea for your husband to take a few lessons on a good moving and well mannered gaited horse first.
That person doesn't even have to go to a professional trainer; someone that's been riding gaited horses for years should be able to at least help your husband get familiar with the motion.
See if you can dig up a long-time, savvy, gaited trail rider and see if they will loan him a horse and go out on the trails with him for an hour