Growing/Spreading Birthmark?

I'm assuming your doctor is concerned about Melanoma. Birthmarks with an irregular border, irregular/odd shape, color variations, and ones that grow rapidly in size are all symptoms of skin cancer. Melanoma is the most harmful skin cancer, but it's also got a very high treatment rate when caught early.

I don't mean to worry you. I honestly don't know that much on the subject, but this is the first thing that came to my mind, as I too have birthmarks the doctors have been keeping an eye on all my life. Melanoma was always their explanation for the concern. I'd call his doctor and make an appointment. If you're not satisfied with the answer you get, see a different doctor for a second opinion.

The other thing that comes to my mind is, has he gone through a growth spurt lately? If he's gained a lot of height/weight from growing (this happened to my brother) his skin will stretch rapidly, and the birthmark won't have time to grow with him, it will just get pulled and stretched out of shape. This will cause it to look radically different than before. We took my brother in when he was about 4-5 with this same concern. (his is on his calf) The doctors ran a few tests and found out everything was fine, he was just growing like a weed. Now that I think about it, I'm betting this is what's going on with your little guy, but I'd still call his doctor, just to get a definite answer and put your mind at ease. I hope all goes well. :)

Get him in and checked out. What I've read, they shouldn't be Growing, but should be getting smaller and less dark.

V.,
I would get the birthmark checked out by a dermatologist right away. I had a large birthmark (that I wasn't born with) that my pediatricians were never worried about, but my mom took me to a dermatologist and they were very concerned and removed it. It turned out to be nothing, but sometimes they can develop into cancer if they are left by themselves. I worked for a while at the American Cancer Society, and while I'm not an expert, generally the medical community is very concerned when skin spots grow and change. I'd get him checked out by a specialist as soon as possible. It might be nothing, but the sooner you catch it the better.

Hi V.,
It is most likely nothing but you should definitely get it checked out. Sometime these things can be cancerous and the best thing for something like that is early detection. A doctor can tell you if there is cause for concern. I suggest having the doctor look at it again and just keep an eye on it. Again it is most likely no big deal. Good Luck.