HI so sorry ur little guy is going thru all this....yes a cord can re-tether after surgery from scar tissue and a trauma to the area....
May I ask, was ur son checked for syringomyelia or chiari malformation?
I also have tethered cord, have not had it un tethered as of yet....but I did have surgery for chiari.....
And what I have learned from reading on this and related conditions it is so important to have the right drs keeping an eye on this and executing the treatment...
Did ur son have the urodynamics testing to see how his bladder is affected?
The other thing that I wondered about was that mayb the knee pain is due to a autoimmune issue like Ehlers-Danlos....something to look into.
We do have a chiari forum and a few of us do have TC....so if u would like to join us in the forum we would be happy to share what we do know....
http://www.medhelp.org/forums/Chiari-Malformation/show/257