Narcotics have the side effect of lowering you bp AND also making you tired...no matter how long he has been on them. Eight years is a long time.
I may not be following what you are telling us, but it seems to me you are saying your husband has low BP and is no longer taking medicine for high blood pressure, this is good. But, it seems he still has low BP, what are the numbers? Is the back pain medication possibly the problem?