Impotence & Erectile Dysfunction Support Group
Erectile dysfunction or impotence is a sexual dysfunction characterized by the inability to develop or maintain an erection of the penis for satisfactory sexual intercourse regardless of the capability of ejaculation. There are various underlying causes, such as diabetes, many of which are medically reversible. The causes may be physiological or psychological.

Personally I wouldn't drastically change his diet so quickly. Something to build upon. More importantly is not focus so much on the diet but on exercising. Exercising has a bigger effect on BP, Testosterone, weight loss and ED than just diet alone.
Two years ago I was in the same position as a male. Wife forced me to the Urologist and he got me all fixed up.
Have you told him how you really feel about this? You need to be blunt and to the point on this issue.
After my girl left because of this I find myself lost. The sad part is I can get an erection on the meds most of the time, but she was so frustrated with the occasions that I couldn't that she left.
All I want now is to find someone who can understand that I am doing everything in my power to make this as manageable as possible and still have pretty good sex most of the time.