Yes he is right,

check these sites out should help you

Causes of Buttock pain - WrongDiagnosis.com

Reasons for Pain in Buttocks - Pain in Buttocks

If it is a confirmed fact that you have Disc Bulge/herniation or posterior bulge and is compressing your nerve roots you could ask your physio to teach you Extension exercises or Mckenzie exercises along with MWM .

check this out dr Mike
YouTube - McKenzie Exercise

Wish you a speedy recovery

God Bless
tc