If your dermatologist
has suggested to come back on September, do exactly as he/she has told you. The changes from atypical to cancer (although) not concluded, are not done as easily as you think. That's why your doctor suggested to wait for this period of time.
With regards to the chance of an atypical mole to be a melanoma
, it is true what your doctor has told to you: wait and see if any changes appear to the mole, especially when it comes to their color (turning black or bleeding), height, etc.
Take things by relaxing as it would help you understand your problems much more clearly.