The answer is "maybe yes" and "maybe no". That's the good news. The bad news is that often the condition, once developed, ir irreversible. Chronically elevated glucise levels cause the progression of both microangiopathy and macroangiopathy. To cut to the chase lesions and deposits form within the walls of tiny blood vessels and the penis is no longer able to retain blood. The result is the development of atheroschlerotic vascular disease (ASVD). Early on, close ccontrol of glucose plus supplements of alpha-lipoic acid may help. Some physicians prescribe a "penis pump" and the blood is retained by means of a gentle rubber-like band around the base of the penis. Ocasionaly there are medications which are prescribed that can be injected into the penis. Slow-release Naltrexane is often helpful, although this is an "off-the-appropriate use" in the PDR. Another alternative is an implant.
First step is he needs to control and manage his diabetes. How or what he's is doing is unknown since you didn't provide us with any information to work with. Take him to a doctor and ask for Viagra or Cialis. Also, try posting on Sexual Health, Erectile Dysfunction, and Mens Health forums.