Tough to say.  There are a lot of pigment variations down below.  Your boyfriend may have diminished pigment on the head and shaft of the penis, making thse look relatively pink.

I can't comment on your other descriptions, beyond saying that they don't remind me of any disease, transmissible or otherwise.

For your own peace of mind, I advise that you have him see a skin doctor, just to get a clear diagnosis (or lack of diagnosis.)  Ask him not to be embarrassed, and to do it for your sake.

This is the sort of question we skin doctors get all the time.  It's part of our job.

Take care.

Dr. Rockoff

A related discussion, Pigmentation was started.

A related discussion, dark and rough skin on penis head was started.

I have this same problem.  The dark area is on the left side of the penis shaft right in the middle(from the head to the base).  It showed up 2-3 years ago and my doctor told me it was a lack of pigmentation (asked about it 2 years ago, asked another doctor again recently).  Doctor checked me out did a urine test said it was negative, but gave me doxycycline incase it was something else.  But now it seems to be getting darker.

Every now and again seems to be a dull pain sensation right running from my scrotum to the dark spot.  Seems like something is going on.   The answer of a normal pigmentation loss may be right but I'm a bit confused.  If it was a loss of pigmentation I would think it would be lighter in color.  The dark color is about the size of a quarter, it is definately the skin, however above the dark spot the skin is a lot whiter than my normal skin color and the underside of the penis head is white.  So to break it down the top half of the head is the good pink color the bottom half is white/pale.  It seems like a vein was injured somehow, sex or sports maybe.
Is the vein theory possible?

If anyone has a similar problem or answer to what is going on I'd like to hear it.