I suspect he now has the results of the MRI, so my comments come a bit late, but I will share what i have learned through this journey.
When some of the tumor remains (residual), regrowth does happen more frequently. There is always a reason why some tumor is left, usually to preserve function elsewhere (personally for me, it was to preserve the facial nerve). If it does continue to grow, the rate of growth varies also, and in some cases its slow enough to just wait and watch. For others, it may be best to take action (I am having Cyber Knife due to rapid regrowth).
There are some great seminars on the ANA website that talk through the options, I refer to them frequently. There is one called "AN Regrowth: A Discussion of Treament Options" and another "Headaches Associated with Acoustic Neuroma Treatment". The headaches transcript has been particularly useful to me as, 6 months after surgery, i have been having some of the worst headaches of my life. Knowing that this is real helps me deal with the new normal.
Your husband is lucky to have you checking into this, asking questions. It can be a lonely journey, even when everyone is there to support, because the symptoms are tough to describe, and they can affect every moment of the waking day.
Hope this helps, and that the MRI shed some light on things.