hello Nit, Is there pain ?, would be usefull to know if there is compromise to Wikipedia reference-linkSIJ function, is he tight in the sciatic dura?, does he complain of pain as you mobilise L5/S1?, does this pain decrease with mobs?. Also what is the distribution of the S1 altered sensation pattern in this guy. These details would be of some help in an evaluation, though on the details so far he appears to have a disc bulge, in which case traction might decrease his symptoms, this might lend credence to the disc being involved. Not an absolute of course but probably worth a try manually.
Cheers