Dad (75) was given triamcinolone + lidocaine for L4-L5 stenosis. He developed weakness in thigh muscles & pain in left pelvis. Is this ESI sideffect?

Possibly. These complaints could be related to the injection and may resolve spontaneously. However if after 5 to 7 days symptoms continue or worsen you need to speak to medical personnel. If your Dad develops fevers or further neurologic issues please have him see emergent medical or ER care. Good luck.