Cannot seem to find the referenced article, but as I recall the regrowth possibility after surgery is only a 5 to 9% chance and most frequently following retrosigmoid (suboccipital) approach. The stats for knowingly leaving some of the tumor would be different. And, of course, different stats for people after radiation treatment.
Did he have an MRI at six months? If so, was it completely clean? If not or he did not have one at six months, you will know on Monday's MRI whether there are any areas of post-op changes or recurrence to watch.
Keep in mind that headaches can be caused from many things besides an AN. Oh, how easy it is for me to jump to any new symptom as being related to the AN! Some are, of course, but some are not. Try to relax until you have the report from this MRI.
Many thoughts and prayers.