My 7mth old baby....

I have a 7mth old baby that was born with a birthmark/mole on his right arm. When he was 4 mths old, his first dr. said that it look like a type of cancer that would turn cancerus later in life with sun exposer. I got another dr. to look at his arm and she sent me to a dermatologist. Around 3 mths of age a light ring around his birthmark showed up, then at 4 mths of age, the white ring started growing. Whit in 4 weeks it has grown over 1 inch - since he was 4 mths it has grown around 3 inches total. It took up 3 mths to get in to see the dermatologist and took a 3 hr drive. The dermatologist has 3 interns with her and would not really talk to me. My baby has eczma (eczema) and the dr said to come back in 3 mths and they will look at it again. What should I do? This thing has been growing on my baby for 7 mths now. Is it dangerous?