Diphenhydramine is the anti-histamine used in Bronchostop cough syrup, singingnutty. Many such old-style anti-histamines used to make you drowsy and so diphenhydramine is really useful in this respect, especially when suppressing and dealing with nasty dry and really exhausting night coughs.
I rarely get coughs, unless it's Flu or Covid-related!, and then my coughing is quite scary! I have some magic syrup for dry coughs which is not licensed for sale in the UK., but syrups like Bronchostop come close. It is, of course, very important to check with the Pharmacist that any over-the-counter medicines are safe to take with your DH's usual medications.**
For rapid relief, there's always the old remedy of steam inhalation. A vacuum flask of boiling water + Olbas oil + plastic bowl and a big towel kept handy can really help to end a bad coughing fit. When coughing starts, just a couple of drops of oil in the bowl, add boiling water and sit very upright over the steam with head and shoulders covered. Breathe deeply through the nose for at least 5 minutes. When coughing subsides, blow your nose and try to sit/sleep as upright as possible against high pillows.
Hope he, and you, get some relief soon.