I'd suggest pushing for a heap of blood & other tests. Unfortunately anxiety is too often treat as MH issues before all physical causes are properly considered & ruled out. The meds given can actually make some possible cause worse too.
The one this screams to me & I know from personal experience & support groups that doctors can be dreadful at spotting & treating, is B12 deficiency. I had many an episode like your DH & ended up in A&E with a suspected heart attack which they then blamed on anxiety- it wasn't, any anxiety was a physical overreaction due to the deficiencies.
POTs, Postural Orthostatic Tachycardia sounds like a possibility for your DHs heart issues. It doesn't show up on normal heart tests, because it isn't a heart condition, though does affect heart rhythm etc. It's a dysfunction in the autonomic nervous system, so basically faulty wiring that affects the heartrate & more. The attacks are scary when you don't know the cause.
POTsUK is a good site for advice & he needs an Active Stand Test as a first line in diagnosis, but print off instructions from POTsUK, to make sure they do the test properly. www.potsuk.org/diagnosis if you have a BP monitor you can check fir yourself at home too.
My surgery messed it up 3 times. POTs is a symptom of B12 deficiency & anxiety is a symptom of both. This link explains it all & why injections are needed & why not to take supplements...
www.b12deficiency.info/
Low Potassium causes panic attacks too, some BP meds can leach Potassium, so something else to look at. Low normal can also make you jumpy. There's also adrenal problems that affect Potassium levels too.
Low iron or Folate, B6 & other deficiency's can cause anxiety too & even a good. Diet might not help if you have an absorption problem, which can have numerous causes, even a very common gut infection such as H.Pylori
I'd recommend looking into other causes first, you might well find he has an easily treat cause to his problems & he doesn't need the pills to get his life back