Hi and welcome to the Chiari forum.
It is possible to develop an issue post op and how long it may happens depends on the individual....the pain he is having can be from scar tissue that may have formed, or from muscles getting tight if he is not doing the neck exercises....it could be he has cerviocranial instability and is becoming an issue the more he is doing...or it could be his activities....
I would suggest he be looked at, when is he due for his post op MRI?
We do have a list of Drs used by the members, u do have to research them and there are a few in TX, I do not know their experience only to say they did treat a member for chiari and the member was happy with them and added the name to our list.
http://www.medhelp.org/health_pages/Neurological%20Disorders/DOCTORS-LIST-FOR-CHIARI-SYRINGOMYELIA/show/1062?cid=186