Thanks for your question on Health Care Magic. 
I can understand your husband's situation. 
In my opinion, he is having anginal pain due to coronary 
artery disease (CAD). 
Following points are in favour of CAD associated anginal pain. 
1. Recurrent 
chest pain. 
2. Relieved with nitrates.
3. Chest pain at bed time in night. 
 Nitrates cause coronary 
vasodilation. So anginal pain will be reduced. 
So better to get done 
stress test (trade mill test) first if you are not affording for 
Angiography. 
Stress test is also helpful in diagnosis of CAD. 
But the gold standard test is coronary Angiography. 
Hope I have solved your query. 
I will be happy to help you further. 
Wishing good health to your husband. 
Thanks.