Hi Just W,
Here's my take on the situation given the limited amount of information. My first thought is this is possibly Erectile Dysfunction (ED). ED is a very broad area with many variations and causes. The two of you with a physician will have to investigate and rule out each type.
First, impotence is not a disease. But a penis that can not get hard CAN BE an early warning sign that something else is wrong either physiologically (body) or psychologically (mind). In order to have an erection it requires the cooperation of the body and mind.
Physiological issues can be heart-related. After all a hard penis requires good blood flow and healthy arteries. Atherosclerosis (hardening of the arteries) and high blood pressure (hypertension) are two disorders to that should be investigated first. Alternatively, is your husband on medications? Some drugs' side effects may affect his ability to have an erection.
Addressing psychological-based causes can be more difficult. The mind depends on hormones to start the process of an erection. Low-testosterone is common with aging and can result in lower libido and erection problems. Fortunately, if low testosterone is the cause, it is easily treated with testosterone replacement therapy. (This is a problem that I had ten years ago and was able to successfully treat my low testosterone which was 10% of normal levels when first diagnosed.)
Another possible trouble area is that the nervous system isn't functioning as it should. Its not able to send a signal to the penis. ED can be an early warning sign of diabetes, stress or depression.
It may well be that it's none of the above, but something as simple as riding his bike! Is he an avid cyclist? There are known cases of ED resulting from the constant and frequent pressure of a bicycle seat on the perineum (the area between the anus and the scrotum).
The take away message that I am trying to convey is ED has numerous causes and your husband should consider himself fortunate. His body is trying to tell him that something is wrong, and he should take the opportunity to discover what is the root problem here (no pun intended).