What Causes An Enlarged Vein On The Side Of The Penis?

My husband is concerned that one vein on the side of his penis has become abnormally enlarged. He is not experiencing any physical symptoms other than the enlargement of the vein - no pain, discharge, etc. It is visible when flaccid but is more prominent when erect. He is highly sensitive about it and I do not see it as a problem or as unattractive. As a result, he has been avoiding intimacy. He kept this secret from me for a week because he was worried and embarrassed. He is of normal weight, non-smoker, rare-drinker, non-medicated but experiencing depression symptoms as a result of being unemployed.