First and foremost, if he doesn't really want to quit, there isn't a setup on the planet that will get him to.
My wife was a complicated case as well. She had the same cough response and complained that even when she could vape cravings weren't satisfied. It turned out that most of that cough response was due to a near constant post nasal drip (PND) caused by enlarged adenoids. Essentially the numbing effect on the throat from regular smoke was able to mask the symptoms, but vaping doesn't have that effect so triggered coughing. If your husband has a lot of PND perhaps he should see an ENT. Smoking can also mask symptoms of early stages of throat cancer so it might be good to be checked anyway.
Also the wife couldn't handle the throat hit from freebase nic, so we had to use salts. Considering Juul seemed to somewhat work for your husband, and it uses salts, that may be what he needs as well. Just in lower concentrations than Juul has.
In order to get the vape to satisfy her cravings we had to mix in some WTA. WTA is Whole Tobacco Alkaloids and contains some of the missing parts from tobacco that some people seem more addicted to. This helps with cravings that straight nic just won't stop.
If you are willing to do DIY juice, we can help with recipes and concentrations. For equipment I would probably start with whatever setup he got from the vape store that somewhat worked. It might help if we knew what that was though. I assume this setup used a refillable tank. It might help if you let him try vaping 100% pure PG (no nic, no flavors, just pure PG) and then try 100% pure VG, to see which one of those by itself is the most irritating to him and then reduce that in the DIY juice as much as possible. If you can rebuild coils and have a dripper, using that could be helpful at this point so you don't have to partially fill a tank with pure PG or pure VG. If not then just use whatever tank you have laying around, but you will need to change coils between using the PG and the VG, or just rewick if it's a rebuildable.
So to summarize what it finally took for my wife was removal of her adenoids and using DIY juice made with salts and a bit of WTA (I do not know of any commercial eliquid with salts + WTA). Now she is a few years smoke free.