My 4 year old has Something odd under arm. Any Ideas?

My Little boy has had this little lump with black on it since he was 2 and he is now 4 and it's still there. Is this just an unusual mole.or skin tag? Has anyone seen anything similar to this before? I'm not asking for a diagnosis just any ideas before I take him to doctors.

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it looks like a pimple you can squeeze and empty it.
but ask a dermatologist if you are unsure