Urticaria pigmentosa

My son just turned 2.  Every time he has been to the doctors in the last year, I have asked about this birth mark like spot on his left arm.  The problem with this brown spot is that sometime it quickly turns read and develops white raised spots within the main red mark.  After a little while the redness will fade and probably within an hour the white raised spots will disappear.  He does not get white spots or red spots anywhere else on his body.  Recently it looks like his brown mark has started to grow a little bit.  The doctors are baffled and one thinks that it could be Urticaria pigmentosa expect for the hive like white spots only occur within the mark.  Does this skin condition sound like Urticaria pigmentosa or is there something else that this could be?  Thanks for your help.