Below knee cap and buttocks pain. Night!

My husband 38 is suffering from lower back buttocks pain ( when sit ) and pain below knee cap , he is continuously following rheumatologist from last 3 years , his Ra anti CCP ANA ENA HLAB27 TTG is negative MRI of si joint and pelvic was normal (2022) Bone scintography ( 2022) normal Every after 2 month rhuematologist call him to follow up ,ESR CRP never raised in 3 year , he have consulted 5-6 rhuematologist but till no one diagnosed it as spondylopathy, presently the rhuematologist treating him as fibromyalgia,BJHS I want to if it is not arthritis, what other spinal or nerve problem can cause pain symmetrical and same intensity and location both side Lowe back Buttocks pain Below knee cap Sometimes heel fingers His pain generally feels in lower limb no muscles pain and swelling in last 3 year , sleep well in night