As Sqad say, there's not enough information here for anyone to come up with a definitive answer but I'll chuck in a possibility (based upon my own experience) anyway:
I was frequently clearing my throat, with an occasional cough, during my teenage years and beyond. It eventually turned out to be caused by a form of acid reflux which doesn't cause heartburn but does irritate the back of one's throat:
https://www.webmd.com/heartburn-gerd/guide/laryngopharyngeal-reflux-silent-reflux#
(I can't recall it being significantly worse when lying down but that would make sense as 'normal' acid reflux tends to be worse when gravity isn't helping to keep the acid in one's stomach).
If you husband has the same problem, simply increasing his daily fluid intake might help to solve the problem (as it did for me). Otherwise proton pump inhibitor tablets (from any pharmacist, without prescription) should do the trick for him.