Question about Arachnoid Cyst

My 8 year old son as a large arachnoid cyst in the posterior fossa.  MRI confirms that it is in excess of 10 cm wide (they did not actually measure at its widest point but it is virtually from ear to ear) and 7 cm deep and it is applying some pressure to the cerebellum.  The cerebellum is towards the center of the brain rather than resting in the back where it should.

He has no neurological problems and is actually extremely gifted.  He does suffer from severe headaches with vomitting about 3-5 times and month and less severe headaches daily.  The first neurologist indicated that we should forget about the cyst and that his headaches are migraines.  This doctor said that arachnoid cysts do not cause headaches and they do not grow.  The second opinion neurologist wants to monitor to be sure it doesn't grow and wants to treat for migraines first (on a new medication because Topomax (topamax) prescribed by the first doctor did not work) and then see if he is responsive.  If he is not responsive to migraine medicine, the my consider surgically treating the cyst.

How do they surgically treat a cyst like this?  If they remove it, will his brain mold back or will there be a large open space?  Is there a concern in the event he has a head injury in the meantime?  Would it be relevant to take a look at his ultrasounds from when I was pregnant with him to see if he had it then?  Lastly, in the event of a severe headache (with vomitting and unable to control the pain) should he go to the ER?  I don't want him to spend 3 hours being poked and prodded on just to be told that it is a migraine but I also don't know if this can rupture or bleed and cause damage?